Go somewhere like Gamespy, register and get it to scan your pc to see any compatible games.
Join a room for the game you want, click join and bob's your uncle.
What games are you thinking about playing? Some games will try to get you to use their own host thing like Ubi.com.
When joining a game look for good pings. That's the time it takes your computer to reply to others in the game and vice versa. Good pings are less than 50ms. Otherwise you'll suffer lag. Lag is where you computer responds slowly and your enemy will see you but you can't see them resulting in your ass getting tapped.