Geeze, some of you people. This guy comes here asking a question, and you just laugh at him. Even if the question is slightly noobish, we were all noobs at some point. Denying this is similar to saying that you hate children, and am glad you were never one.
Anyways, there is no "best 6 Pokemon" to use. It's all up to personal preference, and in no way does a team need to be completely Over Used (best example is where my Under Used Claydol threatened to kill nearly an entire team of Over Used). However, there are basics that, I feel, every team should have. Some of them are obvious, some are less obvious. This is my guide to good team building. So I'll go with the obvious first:
1.) A physical sweeper
2.) A special sweeper
3.) A special wall
4.) A physical wall
Those are explained very nicely by Penguinpal.
The next few are a bit less obvious, and can be filled by the Pokemon above, but generally should be on every well-made team:
5.) A Rapid Spinner - Spikes and Stealth Rock can get highly annoying, especially if you run a stall team where you're required to switch a lot. Rapid Spin solves this problem.
6.) A Hazer/Pseudo-hazer - Nothing is scarier than a Garchomp with one or two Swords Dances backing it up, or a Snorlax who starts Cursing up. Haze (or a move like Roar/Whirlwind) eliminates stat changes, and can put you at an advantage.
7.) A Ghost - Yes, I feel every team should have at least one Ghost type. Not only for the boat load of immunities (Gengar has the most immunities I've ever seen), but it also serves to block Rapid Spins. It highly sucks to set down three Spikes, just to have the foe Rapid Spin it away. However, if you don't use Spikes or Stealth Rock, a Ghost isn't totally necessary.
Now, there are also other types you'll want on your team. Specific types that will put you at an advantage simply because certain moves and types of moves are so common.
8.) A Flying/Levitating Pokemon - Earthquake is the single most common move of physical sweepers. It's absolutely vital that you have something to switch to in order to avoid it.
9.) A Ground Pokemon - If fast or bulky, a Ground type can give you a free switch if you predict an Electric move. Electivire is the best example; if you can predict right, you can catch Electivire on a Thunderpunch or Thunderbolt, and make short work of him, since any kind of STAB Earthquake will likely OHKO.
Hmmm... I was sure I had a number 10. I guess not.
Anyways, that's my brief guide to building a team. It should also be noted that you don't have to follow every step, but it could help. It of course needs to be backed by good strategy and movesets, but I'd say it's a decent place to start if you have no experience whatsoever in the competitive battling scene.