Hi
When you are signed in clic 'user CP' (at the top left in blue)
Then clic 'edit signature' (on the left under settings and options)
Then type what ever you want in the 'edit signature box' (big box, middle right)
You can then clic 'preview signature' (at the bottom) to see how it looks (it will diplay in the large box at the top)
When you're happy with how it looks, click 'save signature' (at the bottom)
Hope this makes sense