I use Photoshop whenever I'm working on new icons / tags for my signature, but if you can't acquire Photoshop for yourself, then there is always Gimp that's free to download and use.
If you want more help on how to make a tag for your signature, then you can always check out the Art & Design forum. They have a great sticky with resources and tutorials on graphics if you need them right here.
Or you could always use MS Paint if you DO use Windows. You can always get pics on the Internet and copy/paste them (if you are allowed to copy the pics, in other terms, the pics aren't copyrighted) into MS Paint, edit some minor things and the background color and save it, you could also add some text! Btw, MS Paint is for simple editing & graphic creation, not for professional use like Photoshop/GIMP/other programs to 'transparify' images, adding 'layers' and so on.
If you find yourself unable to make your own signature or you dislike them, even after some tutorials, there are some art shops that will make tags/banners for you. They're in Art and Design, like Aerilyn mentioned. :3
vacationing in unova for a month! see you soon! ●starboy●
There are a lot of sites that offer pre-designed templates so you can make your own, if not you can always learn how to use paint or photoshop or whatever image software is more attractive to you. Just search on google "forums signatures" or something like that.