[sjard]

Under User Settings, on the sidebar under Settings & Options there should be an option for Edit Signature. It should be between Edit Avatar and Edit Email & Password.

If you mean the bit of text, like Stick in the Mud under my name on the side bar, then that requires a staff member to decide you have earned one. However, they cannot be requested, and are random in when they are given. Additionally, the staff member most likely to have done so passed away a few months ago.