Do you understand the risks of overclocking? Basically you are forcing your computer to work harder than its designed to and as such the biggest problem is heat. If you overclock the components get hotter and can destroy themselves.
Personally I've never tried it as if things do go wrong I cannot afford a new PC and I don't consider the risk worth the increase in speed.
If you know the risks and still want to try it then have a look here as an example click here