How do you use the self timer on a Nikon D3000?

Well your question wasn't specific so I'll tell you how to set it up along with using it.

To set it up, press the menu button on the left hand side of the camera. Go down to the wrench button and scroll down a ways until you see "Self-timer delay." Go to that and select however many seconds you want.

To use it, press the Fn button underneath the D3000 sign. Then, click the button you would press to normally take a picture and it should work.