You need a graphics program. (Photoshop, GIMP are recommended in that order) Then, for first time, I'd recommend looking at tutorials. I believe a member has posted a video tutorial and a text tutorial. Or you can look at the global resources sticky to find some tutorials.
Well, a signature doesnt have to be graphics. If you want it just text (like mine) for whatever reasons, go to the menu bar and go to "user CP" (which stands for user control pannel) and look on the left for "signature." click that and edit it as you will. If you do mean you need a graphic, i would reccommend grabbing one from the many sig shops around here, use that one, then gradually learn to make them on your own.
If you go into the graphics and arts section, then you can ask people to make a signature for you in a sig store. Just show them an image that you want them to incorporate and they will make something for you.
Once you download GIMP or Photoshop, let me tell you that it seems like you will never get used to it. But just keep on trying stuff with it. And yeah, tutorials, especially ones detailing how to use layers, will help a ton.
If you want to know where to go to put the img code so you have a signature go to User Cp ( in the toolbar at the top ) then find signature which is in the toolbar on the left.