First, click on your name at the top of your comment within the thread... then click on "User CP", and then in the left column, under "Control Panel", under "Settings & Options", locate the links entitled "Edit Signature".
Click on "Edit Signature", and add text. Augment it using the hyperlink icon (blue world with a chain link beside it), and the font styles and you can even add a picture using the feature at the bottom of the form (upload file)... just play around.
Once you are satisfied, having used the "Preview Signature" button at the very bottom of the form, simply use the "Save Signature" button to commit your signature to the system.
Hope that helps,
Mungo