Your name up in the top right, click on it and go to "Manage My Profile". You can edit your signature in "My Personal Information" (it does seem a weird place for it doesn't it :lol

.
There is a character limit on the signature that is based on whats written in that box, not the final result. So BBCode will chew up characters. If you want to include a link, I suggest using a URL shortener as full links will quickly consume your character limit. If you want to add lines, you need to add ["*"] (Without the ") where you want the new line to start.