There's two types: World vs. World (WvW) and Structured PvP (sPvP). To get in the first, there's a WvW button in the toolbar on the top left of the screen, towards the right. Click that, and you'll be able to see the current stats and join in. You can do this from anywhere. You'll be able to pick between the eternal battlegrounds (which resets every once in a while), and one of the borderlands (you keep areas you take until somebody takes it back).
To get to sPvP, go into your Hero window, and select the last tab. Then there's a button "enter the mists" or something like that, and it'll take you to a tutorial/setup world, and from there you can pick PvP servers. You'll be set to level 80, given all skills, and given specific equipment in this one.
The difference between the two is that sPvP has fewer players, only two sides, and it keeps everyone on roughly equal footing - your progress in game has no impact on how you do in sPvP. In WvW, there's a lot more players on each side, three sides, and although you're set to level 80, you're limited to your current equipment and skills.