Not to sound like an asshole or anything dude, but seriously, this is one of those things that you should have simply used google for.
Not only would that explain the basic 'what is overclocking' in about 2 seconds, but then as has been pointed out, there are stickies in this thread that link to guides on how to do it.
Even further beyond that, this question has been asked a hundred million times, so even a surch within the forums would have given you an answer.