Definately play around with layers. You spend quite a bit of time just erasing the stuff around what you want to be in a layer, like a single person from a photograph and things like that, but it opens a whole new world to you.
I usually open a picture I want something from, then draw a box around it with the 'marquee tool' (dashed line box), click 'copy'. Then I go to the image I want it placed onto and just 'paste'. PS automatically makes it a new layer. From there I'll erase the bits and pieces around the object I want to get rid of, rescale, tweak colors, etc... To move a layer 'above' or 'below' another one, at the point on the screen where it shows all the layers for the project, you just drag them in the order you want them. There is SOO much you can do, but that is a good basic to get you going hopefully, if paint.net is anything like PS.
I'm not framiliar with paint.net, but I may download it and see how well I like it. The version of PS I'm using is probably a good 10 years old and doesn't do some things very well, like curving text. Yes, there is a way to do it, but its a major pain.
Edit: Just downloaded and played with paint for a minute. Its not too shabby. I'm still very used to PS, but I like some of the stuff that Paint does. I think you could do alot with it.