To make a sig you must first have an idea in mind of what it is you are trying to create. A decent signature will have a background, a subject or main focal point(s), and some sort of foreground effects, lights, or something that relates to your sig's idea. A well designed sig will have good composition. Composition is how everything is placed in your sig, ie: your render or main focal placement can greatly affect your outcome. There is a tip for composition called the "Rule of Thirds":
"Rule of thirds

From Wikipedia, the free encyclopedia

This photograph of a sunset taken in the Thousand Islands region demonstrates the principles of the rule of thirds


The rule of thirds is a compositional rule of thumb in visual arts such as painting, photography and design.[1] The rule states that an image should be imagined as divided into nine equal parts by two equally-spaced horizontal lines and two equally-spaced vertical lines, and that important compositional elements should be placed along these lines or their intersections.[2] Proponents of the technique claim that aligning a subject with these points creates more tension, energy and interest in the composition than simply centering the subject would.[citation needed]
The photograph demonstrates the application of the rule of thirds. The horizon sits at the horizontal line dividing the lower third of the photo from the upper two-thirds. The tree sits at the intersection of two lines, sometimes called a power point. Points of interest in the photo don't have to actually touch one of these lines to take advantage of the rule of thirds. For example, the brightest part of the sky near the horizon where the sun recently set does not fall directly on one of the lines, but does fall near the intersection of two of the lines, close enough to take advantage of the rule." From Wikepedia.


After you have your composition right you will want to create some sort of flow to your sig. Some depth, and some effects will also help create something interesting to call a sig. Tutorials will help you learn the steps and methods to create all the cool stuff you see around. There are tons of good tutorials on this site, just find the tutorials section and get to reading and follow along with the tuts that interest you. or check out www.sigtutorials.com or somewhere with tuts that you like. good luck and hope that helped a lil.