That's really kind of up to you.
I'd choose a picture you want to use for the background then choose a character or multiple characters for the main focal point.
The easiest way to make a signature is to simply find an image you like, crop it to a size that is sutable for a signature and add some text to it. The text can be anything most often your username, sometimes a quote sometimes both. Then add a border if you fancy. You can do all of this in MS Paint or preferably Photoshop which is what most people who are remotely serious about making sigs use.
I would suggest looking at other members signatures to get some ideas and maybe browsing through the
tutorials section for some neat tips and tricks.
I'm presently working on a tutorial on how to make a simple signature, but I haven't even started it yet. I want to get it done before the end of this week, but things might come up in the mean time. I was only going to post it at this other forum I frequent, but since it will be benifical to new members here as well I will also post it here when I get it done, so if you can wait a few days you'll have that as a referance.
After you've made a signature you have to upload it to an image host I suggest either Photobucket or ImageShack, both are free. After that you go to your control panel on the forum, click edit signature, click the IMG button and insert the URL of your signature and you're all set.