Honestly?  If you have a script, then carry on working on that.  There are probably lots of people out there with the proper drawing skills/tools who would love to help make a cartoon. 

At the very least, upgrade to Paint.Net as it is free and MUCH, MUCH better than paint.  It does layers, so you can have text on one, trees on another and the bird on another. Then you can show and hide the different layers as necessary.  Also, use text tools for text and line tools/shapes for objects.  It is impossible to to draw with a mouse.  If you use thicker lines they will look less blocky and Paint.Net will also smooth them out as well.