First ready the sticky Signature Rules thread in the chit chat forum.
Then you need to draw one up or ask a friend thats good with graphic design programs to make you one. Or there are a few sites that make them for you for a fee... just make sure to tell them our rules so it doesnt come out to big. If you want to try to make one your self a good fairly inexpensive graphic design program is
Paint Shop Pro or if you have alot of money to blow you can get adobes photoshop 7 (what I use). There are also some free ones like The Gimp however there not as easy to use
Once you have made your graphic signature and saved it in jpg/gif/png format you need to find a place that will host it online for you. I've always had my own web servers so I've never needed to find a signature host. Hopefuly some one else has some good places to check. If your guild has a webpage they might be willing to host your signature.
When your signature is now on the internet just place the code show below *with no spaces* in your signature:
[ img]http://www.myhost.com/mysignatures/sig123.jpg[ /img]
Thats it.