Ah yes, a signature. Click User CP. Then on the left-hand side there should be a heading that is something like Edit Settings... under that there will be Edit Avatar, and then Edit Signature- click on that and add your signature

You can also click 'Quick links' on the light blue toolbar at the top of the page and select 'Edit Signature' under the User Control Panel section.

As these suggestions should answer your question, I'm going to close this thread now. If you have any other questions or issues with using the website then feel free to make a new thread or send a PM to a buddy who should be able to help you with navigating the site.