The standard infantry are good against air units, the anti-tanks are good against tanks and the snipers are good against other infantry. Use forests, buildings and different cover at all times. Usually the other team underestimates infantry, thus making it easy for you to succeed :3
And just go into a server and try out stuff, don't take it too seriously, try each branch out and see what fits you best. I'm best as Armor and infantry. It's very fun.