Is this a prebuilt computer, or one someone else built for you? By prebuilt I mean Dell, HP, Acer, Gateway etc. If so, what is the model?
Overclocking is making a component run over the speeds at which it is sold to be at. If you don't know what you are doing, don't even attempt it until you have a very, very firm idea of what you are doing and what it does. I wouldn't even try to overclock an onboard GPU, if indeed you can at all, not only because performance gain would be next to 0, but also because of extra heat build up. The heatsinks on the chips aren't designed to take large amounts of heat, and when done properly, overclocking won't throw out substantially more heat, however that bit extra, I wouldn't personally be comfortable with