Hover the mouse over the "My Gaia" tab (below and to the left of your avi in the header), then select "Account" from the drop down menu. On the right side of the screen, there is a list of links. Select "signature." In the box you can type words or use BBCode for pictures, colored text, etc. Just make sure it's less than 500x500 pixels (mostly for pictures) and there's a 255 character limit for text minus URLs. When you're done you can preview it or just save it.
Sorry if that's overly simplified.... I find it's easier to over-explain rather than under-explain. >.<