I use photoshop for mine, but thats a pretty expensive purchase just to make some sigs. Maybe there are some freewares out there that can do something similar. I know you can use Paint, but it's not as easy to use and there aren't half as many options but it could be a start if you happen to have that on your computer already.

one tip:

1. do not steal artwork. I used to before I knew it was wrong, but you can make people really really angry with you. Use your own art or use screenshots of your character. It's easier in the long run.

feel free to ask any questions. There are alot of sig makers around here.