You don't need anything in general. Different games uses different mods, and the players usually provide the links to the material you will have to download to play in that game.
If you want to play in a game, just post in the appropriate thread and make your presence known, then the GM or the players of that game can look up what open countries exist and so on.
If you're unsure whether your schedule fits the game's, look in the first post to see what time the game is set to. And don't be intimidated to post; nobody will mind and the worst thing that could happen is that there is no open spot. But every game will need a sub eventually.