You are allowed to copy music for your own use between your computers and/or iPods, and to burn a copy to a CD for safe keeping. You are not allowed to redistribute by selling, sharing or giving away music.
iTunes
It won't copy if you manually manage music. Copy the MP3s to your hard drive and then import the folders in ITunes.
Copy CD Music to localhost and convert them to iTunes compatible audio.
You can buy a legal copy on iTunes.
updating your iPhone should not affect your music, but if it does, you should have a copy of your music stored on your iTunes authorized computer. If you purchase your music on iTunes, you should have a copy of all of the music you ever purchased in iCloud incase you lose anything. I think manually managing your media on iTunes has something to do with losing your music after updating.
itunes
Finder >Your home folder > Music> iTunes > iTunes music. Depending how big this folder is, copy it onto a flash drive/external hard drive and paste it on to your new Mac, then go into iTunes and import the music folder. Should work. Good luck and congrats on the new Mac!
Garageband is your own music. You are making your own music. If you mean someone else's music, you can copy music from Itunes when you create a new song. Select + (Create new song) and then Copy from iTunes at the Song selection page.
on PC it is file>import to itunes on mac its preferences > import to itunes click on what you want
For "legal music", itunes is the best.
Bearshare, Itunes and Limewire.
iTunes.