You can set and change your signature by going to your username in the upper right corner > click to get the drop down > select User Settings. On that page, select the Account tab and then go down to the area that says "Conversation Detail Options". On the right side, you will see the option "Edit Post Signature". If you click that link, you will get a popup editor that will allow you to enter/edit your signature.
This option may be disabled by the Administrator. The Administrator may also have restrictions on how many lines or characters may be in the signature.