You must be an upgraded member of the forum before you are allowed to use a signature.
Your signature shows on the About tab of your profile and directly underneath the first post you contribute to any thread.
You may post 2-3 lines of text or 1 banner, 728x90 pixels max. Do not use a font size larger than 2, crazy fonts/colours, sound clips, excessive smileys, or anything else annoying.
Find out how to create your signature in the Newbie Guide.