Perhaps you have used "Paint" in Microsoft Windows? This program is made to mimick, but improve on Microsoft's "Paint." If you have any experience in "Paint", you have experience in "Paint .NET" as well.
No experience is needed. Just find something to edit, and start editing. Or, simply start drawing.
Use the pencil tool or the brush tool to draw and "paint" on the screen. The eraser tool erases. Text makes text. The Fill, or "paintbucket" tool fills in enclosed areas with the color specified in the color wheel (on the left). The eyedropper tool picks a color from the image and makes it the color you will draw with. The shape tools make shapes. The selection tools allow you to select areas and then move them, delete them, or use the boundaries of the selection as limits on where you want to color or erase.
Those are the ultimate basics...to learn more, play around with the program and read the help files like Mr. Brewster has said.