At some point you may find that you don't want all of your music stored in the Apple Lossless format and may want it in MP3 (or vice versa). Although there are programs that will do this for you, it can also be done from within iTunes.
As the programs are slightly different for Windows and Mac, I'm going to give you the basic instructions:
- In the preferences, go to the area that allows you to change the options for Importing (usually under "Advanced")
- Change the codec (usually selected in the "Import Using") to the one that you want to convert to (i.e., if you wanted to change a bunch of songs from M4a to MP3, you would select "MP3").
- Save those settings.
- Highlight in the library the tracks that you want to convert.
- Go to the "Advanced" menu and you should see any option that says: "Convert selection to <codec you selected>"
Sure, there are other methods, but if you're already using iTunes to manage your music, this one is pretty damn easy.