How do we know if you aren't going to make another furry avatar?
This is the help section.
You need a photo editting program such as photoshop, paint.net, incscape, gimp, etc.
Then you can probably find a tutorial on one of their forums to do transparency. Its as easy as deleteing a selection though or apply a transparent color to the area for those programs.
Paint.net isn't MSPaint. MSPaint (the paint that comes with window) cannot do transperancy.