First of all click "Profile"
Next look on the left and click "Forum Profile Information"
Scroll down and find the "Signature" box. In this box you enter the URL of your signature in the [ img ] tags. For example in my signature box it says:
[IMG]http://i13.photobucket.com/albums/a278/djnutta/ShanaWIP2.png[/IMG]
If your signature is on the computer then you need to upload it to a website such as imageshack or photobucket.