The biggest piece of advice I can give you is to not use Paint's shape tool. It's mostly OK to use the curves and lines. However, circles, rectangles, etc. make any work feel like it was built by a machine.
Also, if you're working with a laptop and use a mouse pad instead of a regular mouse, don't. You need to have an external mouse with a mousepad (not necessarily store bought, the back cover of a notebook works fine).
As Zamcio said, practice and experiment. Those are both crucial to your development.