Websites such as Pandora and Last.fm are great for finding music you like. You'll hear familiar tunes and also be exposed to new material. Blip.fm is another site that allows you to search for specific songs. You can also listen to music posted by people on your friends' list.
Enter your musical preferences. Pandora and Last.fm will ask for a list of artists you enjoy. You can also rate music selected for you. With Blip.fm you enter songs and artists and are provided with a playlist.
Think outside the box. Keep music videos in mind. If you can't find a particular tune, remember that lots of record companies have music videos on social video sites such as YouTube or Daily Motion. These sites are also great for listening to live performances, though sound quality varies.
Plug in your headphones. Listening to music at school can be helpful, but it's unlikely others will want to hear it. Always use headphones.