How often: Generally you should try to clean your computer every 3-6 months. If you use your computer often and if it's in a very dusty environment then you should clean it sooner (every 3-4 months) if you use your computer sparingly in a fairly dust free environment you don't need to clean it as often (every 5-6 months).
What you need:
Screwdriver (to open up the computer case)
Can of Compressed air (to blow away dust inside the computer)
Cleaning Liquid: Rubbing alcohol or mild cleaning agent (dish detergent mixed with water...).
Cotton swabs (Q-tips)
Paper towel or cotton cloth
Make sure your computer is turned off before you begin the cleaning process!
Notes 1. A computer on the ground attracts more dust than one elevated on a platform.
2. Static attracts dust. Wiping down your computer with a solution of one half vinegar and one half water will minimize static.