You can increase your rating by winning rated games. You can check out this link to see the things that can make games unrated. If there is nothing that makes a game unrated, it is rated.
You generally gain more rating when you win a game with a lower player count (ie: winning a balanced 1v1 will normally give you more rating than winning a balanced 6v6), and you generally gain more rating when you win a game where the team ratings are not balanced in your favor (ie: the 2 teams have similar overall ratings, or your team has less overall rating than the enemy team does).
It is better to try to improve at playing the game than to try to game the system.