I try to pick a champ that rounds out my team. If they're all AD melees, I might take Heimer, Gragas, or Maokai, to defend a point with ranged magical AOE. If everyone's picked mages, I might take an assassin or hard carry like Poppy who can get the job done with some sustained DPS.
The couple of basic tactics I try and stick to are...
Hold 3 points: your side of the map + top. Don't, DO NOT try to capture anything more than this. You'll get killed trying, and you'll lose top or bot for your trouble. The exception is if you've just aced them or something like that. All you need to do is hold one more point than the other team and you're winning.
If you're defending a point, fight as close to it as possible. Sit on top of it. Kite enemies around it if they dive you. This lets the tower do its work. If you run off it to chase someone for a kill (you greedy-guts) you'll probably get 1v2'd and die.
If your team isn't doing these things, try suggesting them. "Let's defend these 3, don't go for any more captures."