How can I organise my iTunes library?

It's a mess. I didn't choose to put a complete mixture of albums, songs, radio podcasts, MP3s from CDs, all wrongly classified, most of which I don't want to carry around. I can't work out how to actually move anything. Is it necessary to download the entire contents of a complete album when I only want to listen to a few tunes? Where do radio programs downloaded from the BBC fit here? Why do I have to use iTunes? I don't understand hows it works. Is there a place I can read about it?