Take things slow and steady, don't rush into things. There are many guides available on this, but remember that overclocking is not without risk. Overclocking relies on the fact that computer components have a degree of tolerance built in due to the manufacturing process. Be aware that not all systems can be successfully overclocked, and that seemingly identical systems built with identical components might not be able to be overclocked to the same extent as each other.
Before you overclock, read the motherboard manual and ensure that you understand the various options in the BIOS and also how to reset the BIOS to its default settings if you make a tweak and are subsequently totally locked out of your system (the best way is to remove and replace the motherboard battery).
Start by resetting the BIOS so that everything is running on automatic settings. Whilst you are there, disable any hardware that you don't have or use - e.g. com ports, parallel printer ports, floppy drive (if your system doesn't have one) - to free up their resources. Make sure that your system is able to run stable at this setting before attempting to overclock.
When you are satisfied, you can begin overclocking. Be aware that overclocking means running components at a higher frequency (and, in many cases, a higher voltage) than normal and that means greater heat is produced. You need to ensure that your cooling is up to the job. A digital point-and-read thermometer can also be useful for checking the temperature of overclocked components.
Being slow and methodical is the key to a successful overclock.