Dont start off on the wrong foot like I did. I would get Corel Paintshop pro. Get the 30 day trial, and try to save up some money for it.
Composites are near impossible with Paint.net, so if you dont want to pay for a payware, I would atleast get GIMP. With GIMP you can use the magic wand tool to get rid of the background, and with paintshop you can just erase the background with a simple tool. A major mistake i used to make, is that you should never make a cut-out after you have resized the picture. Always make the cut-out and the resolution you took the screenshot at, and then resize it.
As for tips, just try not to over-do any of your edits. Dont put so many filters to the point that you can barely make out the plane anymore ( i did that once or twice

). Other than that, I would love to see what youve come up with, and the guys here can help you out from there.
Nick
Corel Paint Shop Pro Photo XIGIMP