This article will teach you on how to be completely silent, absolutely dead silent when doing anything, as silent as a ninja.

[edit] Steps

Listen closely to every sound you make. Be aware of your surroundings and how they will act when you come in contact with them. If you are trying to walk silently you don't want any floorboards creaking or twigs snapping.
Practice a lot. You must learn how to blend in with the surrounding sounds.
Have confidence in yourself. The moment you get scared you will fail.
Wear clothes that don't rustle. If you are sneaking around in the house and you have wood floor boards wear two pairs of socks.

[edit] Tips

Try not to breath heavily either.
Practice sneaking up on animals. They are extremely sensitive to sounds, so if you can sneak up on a cat or dog you know your training is almost complete.
Try to breathe through your mouth because you can inhale more air with less sound and time.
Try not to swallow saliva when too near a person as you may create a "gulping" sound, revealing your location.
If you can hear yourself then the person/animal that you are sneaking up on can hear you too! So be careful.