Hi Paddy,
First, you will need a picture. A banner style signature picture, say 500px by 100px.
You make this with your favorite program, like PS or PSP or just MS Paint.
Then, you will need to host it somewhere on the net to be able to link to it. You can use your own site, if you have one, or an image hosting site, like Photobucket (http://www.photobucket.com).
Then, you just make/take the IMG code, and paste it into your signature box in your forum profile, and it should work.
Most image hosting sites produce this IMG code for you, but if you host it yourself, you just take the URL and put (without the spaces in them) [ IMG] in front of it and [/ IMG] behind it.
I hope this helps!
Regards,
Fred