Most games make it really easy to play against other people online. There will probably be a “Multiplayer” option on the main menu: just click on that and follow the instructions. You may get a choice of playing “co-op” (with your friends against computer-controlled enemies) or “competitive” (playing against other people).
At LF.Group you can find new teammates in your favorite online games. No matter what games you play mostly - there are definitely people who perfectly match your time slots and in-game goals.