Hello! Welcome back!
In the upper right corner of the page, click on your username next to where it says "Notifications", then click on Settings, then on the far left side of the page, under "My Settings", you can edit your signature.
Good luck! =)
In the top right corner of any Enjin-powered web page, you should find an 'account' section. Navigate to this area, and look for "Signature." It's in rather large text, so you should have no trouble spotting it. You should then see a display of your current signature (In your case, nothing.). Once you click 'Edit,' however, an editing box will fill the screen, allowing you to change what's in your signature. The signature is BBCode powered, so make sure you don't try to use HTML or anything like that. Once finished you may either save or cancel, and the changes normally take effect immediately.