Okay, so Paint.net is ine of the many freeware that is almost as good as Photoshop. I used it when I was doing sprites, and i found it very useful.
As for spriting, i feel itll help you to go about it this way:
1.) Set up the parameters for one separate sprite
2.) Make the outline
3.) Fill in the base colors
4.) Add shadows (and highlights, if needed)
5.) Add other specific details if needed
I hope that will help you. Happy spriting!