Signature sizes should be 600 pixels wide, 125 pixels deep and not more than 50kb file size.
Create them any way you like in a graphics program, or provide some information on what kind of thing you want in it and someone will probably make one for you.
Make yourself a free account at somewhere like photobucket or imageshack, upload your signature to it through their page
go in to 'user CP' at the top.
From the side panel choose 'edit signature'
then put the url to your signature between image tags and save it.
something like: (img)urlhere(/img) but with square brackets instead of parentheses.