Click on "my profile" near the top of the page (just under the "post new topic" button). Then click "edit profile" near the top of the profile page. About half-way down that page (under "Profile Fields") you will find a place to create your signature. All formatting options that exist in regular posts (links, bold, italics, ...) work the same way for the sig. Before you select the "Update Profile" button at the bottom of the page, make sure the "Check 'Show Signature' by default?" box is selected at the bottom of the page so that your sig will be automatically added to your posts.