If you truly want to get into GFX, you have to get a feel for the program. Know what effects do what to your signature, know how to make it blend. This can take a very, very long time, but there's a couple ways to speed up the process.
Look at some basic tutorials first. I can already see there's been some posted above. Once you've got the technique mastered from a tutorial, keep that technique in mind when you start making signatures later. Progressively move on to harder tutorials, before you know it, you will know where buttons are and don't need to specifically know how to get to them. After awhile, stop using tutorials, and start incorporating the different techniques that you found in your tutorials to make a signature.
You also have to know what looks good. If you're going for a grunge signature, make sure the color is something that can go with that, and the render. An entire signature can be completely ruined if you do not apply the right effects to match your setting. Also, one very important tip, one that I absolutely cannot stress enough, is make sure, if you are using an image/render with your signature, that it actually blends with the signature. Time and time again I see renders in signatures that are merely placeholders in the signature, they're there just because they have to be. If you really want your signatures to be a canvas of your imagination and actually get your point across, make sure your render blends with your background. There are many ways you can go about doing this, and in time, you will figure out these techniques and put them to your advantage.
Sorry if I didn't put it all out in front of you, but graphics making can be a complicated process, and if you don't have the dedication to take a few months to truly become familiar with it, there's not too much hope for you. Baby steps, remember. Take your time, you don't want to go too fast, or else you may rely on one technique too much and all of your signatures will be the same story, just a different image. Always keep stuff like this in mind, and you'll be a skilled designer, making very well designed banners for clans and whatnot.
I know I don't look the type of a GFX designer with my signature not being all graphically-impressive, but just trust me on this one.