Go to DeviantArt or something and search for "sig tut" or sig tutorials, they help a goddamn lot. I remember when I first started doing graphics / design, I had no idea. But seriously man, get around the tutorials.
Some basic pointers, though:
- Get a render. This is basically a picture that you will use for the focus of the sig. Common renders are characters from tv or video games.
- find the flow. Flow is important in a sig, it's basically the direction that the eyes follow. Say your render is holding a sword - make the flow the same as his sword direction. Then, make all lines / effects follow this direction as well.
- Choose suitable colours. Common colour layouts are - red / orange / brown, blue / purple / white, green / brown / black. You can look at any colour wheel and the colours that are opposite go together.
- Put text in a suitable spot. Eg, next to the focal point (usually the face / head of the person in your render). Put some lighting on that point as well.