1. Get an program.
I suggest Photoshop but if you dont want to pay, i suggest GIMP or Paint.Net.
2. Find a good Render
A render is basically an image cut out. depending what you want your sig to be about(theme)
3. Get some C4D and Fractals.
Basically just effects.
4. Practice
Just mess round with your chosen program, find what you like and put it all together with the Render, C4D, Fractals, Etc.