There are tons and tons of applications to tag music files... Some have GUIs some are command-line some are smart some are dumb, but as always we'll use the dumbest command-line application because we'll learn the most about what we are doing that way.

To tag MP3s we'll use and application called id3ed. It should be provided with most distros, but if not, visit the previous URL to download and build it from source.

Now to start with id3ed we'll run the command

id3ed -i sample.mp3

on the sample.mp3 file I've provided. Our output should look like this

sample.mp3: (no tag)

because this file hasn't been tagged. We want to add a tag to this file, so we'll run this command

id3ed sample.mp3

and you will be prompted to fill in each field of the tag. Your output should look like this

File sample.mp3: (no tag)
songname[max:30]: Arrive
artist[max:30]: Gaim Developers
album[max:30]: 
year[max:4]: 
comment[max:28]: 
tracknum[max:3]: 0
genre[0-255/name]: Sound Clip

Now, if you don't want to fill in a particular field, like I did on album, year, comment and tracknum, you can just hit enter to bypass those fields. Another tip is that if you aren't familiar with the available genres, run

id3ed -l

and you'll get a formatted list of valid genres and their associated indexes.