You need something like GIMP, or photoshop. You can make it through paint, but it won't be anything eye catching.
But an MS sig? If you're going to want to learn how to make a sig, you shouldn't try for an MS sig of all things. I'm not a total expert, but knowing how they're oftenly made, it's best that you try something else.
If you have photoshop or anything, google sig tutorials.
And lol, charging sigs for money. If it was a drawing or some awesome wallpaper then that's understandable. But to pay for something so small is just a joke.