If you're completely new to Supreme Commander and or want in-depth explanations of basic FAF concepts, you may want to check out the FAF Guide before proceeding.
Unless you have a better idea, start the game with a basic build order (BO)
a. Use your armored command unit (ACU) to build a land factory, then 2 t1 power generators, then 2 t1 mass extractors (mexes), then another 3 t1 power generators, then another land factory. Have your first land factory make a few engineers before making anything else, and use your engineers to build mexes and get reclaim.
b. Alternatively, if you start near a hydrocarbon spot, you can use your ACU to build a land factory, then 4 t1 mexes, and then have it help build the hydrocarbon power plant, and then have it build another land factory. Have your first land factory make a few engineers before making anything else, and use your initial engineers to build the hydrocarbon power plant and then build other mexes and get reclaim.
Scale your economy over time
a. Make and upgrade mexes (make many t1 mexes first, then progressively upgrade mexes to t2, then progressively ring t2 mexes with mass storages, then progressively upgrade mexes to t3)
b. Make more power generators, transitioning to higher tech power generators eventually.
c. Make more build power (more engineers, and more factories and or higher tech factories) - it's often a decent idea to have 1 factory making t1 engineers for most of the game (you can use those engineers to build, assist your factories, assist your mex upgrades, get reclaim, etc).
Spamming units is often worthwhile, but don't forget to transition to higher tech units
a. Spamming t1 tanks is very useful on a lot of maps and is something you should generally do, but by sometime around minute 8-12, you should generally switch to building almost entirely t2 units on a lot of maps, and you generally should transition to t3 units by a few minutes after that.
b. You can generally spam interceptors against t1 and t2 air, and spam ASF against t3 air.
c. Navy is a little different in that making frigates is generally a good idea whether it's early, mid, or late game. However, you should generally still transition to making t2/t3 navy, while continuing to make t1 frigates.
Balance your economy
a. Avoid stalling energy (don't get to 0 energy storage with a negative net energy income).
b. Spend your mass (if you have a lot of mass in storage, use it, and try to preemptively use your mass so that you avoid getting a lot of mass in storage in the first place).
c. Avoid building too much energy (ideally, you generally don't want to make any more energy than you'll actually use, but in reality, you should generally just try to avoid making an excessive amount ((ie: try not to have more than like +100 net energy income by before minute 5, +200 by minute 10, +500 by minute 20, etc)).
d. Avoid stalling mass (this is less important than the other 3 things listed above, but trying to spend mass that you don't have is generally an inefficient use of your build power compared to things like reclaiming with your engineers).
Expand around the map right after your initial BO
a. Keep building mexes throughout the map/game with your engineers.
b. Use military units (especially tanks on most maps) to take mexes/territory, defend your units, kill enemy units, and raid enemy mexes/engineers/etc.
Use your ACU
a. After your initial basic BO, you should generally make 2 more t1 factories and 2-4 more t1 power generators with your ACU in your starting area before moving your ACU to another area.
b. You should generally then use your ACU for securing expansions (groups of mexes) and engaging in combat.
c. Don't be too reckless with your ACU. It can be very useful in combat, especially with a gun upgrade, but you should pay some attention to it and generally try to avoid accidentally suiciding it.
Scout, scout, and scout some more
a. Seriously, keep scouting throughout the game (most people don't scout enough).
b. Pay enough attention to what you scout.
Take favorable engagements
a. Generally take battles you are likely to win (ie: your army is sufficiently stronger than the enemy's army and is not countered by it).
b. Generally avoid battles you are likely to lose or inefficiently trade in.
c. Taking the right engagements and avoiding the wrong ones is generally especially important in air.
Focus on the right things
a. Don't play too zoomed in - things like perfect adjacency and ideal base layout are just not that important compared to a lot of other things.
b. Generally, you should focus on the macro game more than the micro game overall, but still micro some.
c. Manual reclaim is generally not considered a good use of your time after the early game, aside from stuff like the occasional 1k+ mass value wreck
d. Generally try to avoid having idle units, especially in the early game, unless there is a good reason to have those units be idle. However, don't worry too much about this. Using your units matters, but so do many other things.
e. Setting up and using good hotkeys can help you save time to focus more on the game and less on your mouse. You can generally access the in-game hotkey menu by pressing F1 (you can launch a sandbox game where you can test and set up your hotkeys, so you don't have to edit them during real games).
f. Focus on doing the best you can and learning what you can do better instead of blaming other player(s) for what they did/didn't do.
Counter the enemy
a. As long as you scout enough, you should know what the enemy makes, and thus, you can generally try to counter it.
b. So, for example, if the enemy makes a bunch of land units, minimal AA (anti-air) and no planes, you can make bombers and gunships to fight them.
c. If the enemy makes a bunch of pd (point defense), you can generally make units that go elsewhere without getting in range of the pd, or make units that outrange it (such as artillery or missile launchers), or eco a bit more to gain an economic advantage.
d. If the enemy spams t1/t2 air, you can generally make interceptors and mobile flak (t2 aa).
e. If the enemy eco's a lot more than you, you can generally attack them and take a lot of winning battles and gain map control.
f. If the enemy turtles hard with minimal map control, you can generally just take the rest of the map and out eco them for a while and eventually kill them pretty easily.
g. If the enemy acts super aggressively, you can generally try to bait them into bad battles (for them) and then get the reclaim from those battles.
h. So, generally, as long as you scout enough, you can try to counter what the enemy builds, how the enemy plays, and where the enemy sends its units.
a. You should generally use attack move orders on engineers for reclaiming. Single attack move orders sent directly from the factory producing the engineers are especially useful, as they allow the engineers to have extra reclaim range, and those orders can be moved around the map via shift-click and drag.
b. Manual reclaim can be useful during the early game if you have nothing better to do.
c. Patrol is generally worse than attack move.
d. It is generally very worthwhile to use t1 engineers with attack move orders to get the reclaim from the wrecks from battles that are fought during the game.
e. Fighting battles where you can get the reclaim matters a lot, and feeding reclaim to your enemies is something you should generally try to avoid, unless the amount of damage you would inflict would have a greater impact than the amount of mass your units' wrecks would donate.
f. A wreck from a dead unit can yield up to about 81% of the unit's mass cost.
g. It can be worthwhile to kill/reclaim your old low-tech units to have additional mass to put into new high-tech units.
Think about value and opportunity costs
a. It can be useful to think in terms of equivalencies based on cost. For example, if a t1 tank costs about 50 mass and a t2 mex costs about 900 mass, would you rather have 18 t1 tanks (900 / 50 = 18) or 1 t2 mex.
b. Lower tech units generally do more DPS (damage per second) per mass, but higher tech units generally have a lot more health more mass. In mass-equivalent fights, higher tech units normally win in practical situations by large margins.
c. Upgrading a factory costs mass that could've been put into units, potentially resulting in a temporary disadvantage on the battlefield. So, upgrading earlier can be a short term vulnerability for a later strength, while upgrading later can be a short term strength for a later vulnerability.
In team games, be a team player
a. If you need help or expect to lose your lane soon, say so
b. If you don't know what to do next, ask your team what to do
c. If you have extra mass and can't spend it fast enough, you can gift it to a teammate who can
d. If your opponent is attacking your ally, help your ally or be doing something sufficiently worthwhile to cover the cost of your ally's additional hardship
e. If you notice an enemy T4/nuke/etc that your team hasn't noticed, ping/mark it and say something
f. Try to coordinate your units with your allies' units when it seems worthwhile
Use helpful UI mods
a. You should probably try UI mods like NoShake, Advanced Target Priorities 1.1, Rheclaim, and Supreme Score Board.
b. You may also want to get an icon mod, such as EternalStrategicIcons or Penguin's Icon Mod.
c. You can search on the vault for numerous other UI mods to try, and you may want to check out things like HotBuild Overhaul, Additional Camera Stuff v3.3, UI Party, Common Mod Tools, Selection Cost UI, Reminders, Selection Deprioritizer, Disperve Move, and more.
d. Note that some mods require other mods, such as Common Mod Tools or UI Party, to function properly, and that sort of information is generally specified in the mods' descriptions in FAF's vault.
Watch replays of your games to learn what different players did well/poorly (especially look at what you did and at what the highest rated players in the game did)
Get additional advice if you're stuck or confused
a. You can ask for advice in the gameplay-and-training channel on the FAF Discord.
b. You can also ask for replay analysis (of a game you recently played) there (trainers generally prefer to go over 1v1 replays rather than like 6v6's btw).
c. You can also find a trainer here.
d. You can find additional guides and other helpful information here.
How To Play FAF & Get Better - Cliff Notes
pfp credit to gieb