to make a banner...well you can do it a couple of ways.
First you start out with a picture. either one that you have constructed yourself or one that was already made for another purpose. You could also just edit another picture.
Then what you do is upload it onto the internet. You could use photobucket.com or imageshack.com or another similar site.
After that it is the simple step of taking the pictures url and putting it in

(image brackets) in a designated place, either your signature or on a forumn.