Help with iTunes

I understadn how to change song titles but if there isn't an artist on iTunes then how do I add one. Ie the song is called Haiti by Arcade Fire. but when downloaded it ends up being Haiti_Arcade_Fire as the title. With no artist or album, so HOw do I change it so that it says Haiti under title, and Arcade Fire under artist etc.

Highlight the song (or songs), and choose Get Info from the File menu. In the resultant dialog box you now have control over all ID tags.