W.O.G.L is the only mainstream league for combat arms at the moment. Basically they have seasons every few months where you can register a team and then compete in a tournament for prizes. However its not that easy. There are many dedicated teams that have been playing combat arms since it was in beta and they will know every trick on every map. So if you want to join, be prepared and if you want to win, make sure you put in the effort to know the maps as if it were the back of your hand. To practice with pros, there is a specific server on combat arms you can go onto but I haven't played in some time so just look for games named wogl pub.