"I try and I try and I try, but EVEEEEEEERYBODY WANTS TO PUT ME DOWN!" -Somebody to Love by Queen.
But seriously, how do you make a good signature when you only have MS Paint?
I usually do a google search and find something that will fit in my signature.
There's also art shops on here that do banners for sigs.
If you really wanna make your own, I would recommend a program called Paint.NET.
Paint.net is a good tool to have if you don't have the likes of Photoshop. Try GIMP as well.
Those things are somewhat harder to use, but you should be able to get the hang of it once you know what each tool does.
Other than that, follow @Doctor jokool's advice - that should get you started in the imagery department.
There is no possible way to make a good signature with MS Paint. 100% impossible. You'd have better luck finding Atlantis. Get Photoshop.
Or pirate it ;P.
...Or check out my shop ^^;
I say "screw it" and type some random crap in my sig, then change it every once in a while. Signatures don't have to have banners~