Do you want the tracks to have the ame volume or the same loudness? These are two completely different things
You achieve maximum volume by "Normalizing" the song during export. This ensures that the peak level of each song reaches (the same) 0dBFS. You can set that in the GarageBand System Preferences
This is the "percieved" volume/loudness of a song. Two songs could have the same peak level but one may sound much louder than the other. This is part of the art of mixing and mastering, to know how to use compressors, limiters and equalizers on single tracks and on the master track.
Hope that helps
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Normalizing doesn't help a lot in the case of a "mixtape" since it increases or decreases the level of all the tracks in the same way.
I see two ways to achieve what you want, assuming you put each song on a different track:
1. Adjust the individual track volumes, trusting your ears.
2. Leave the iTunes step out altogether (what is it good for anyway?), collect all the audio files in one place, open them one by one in an audio editor, and normalize them to 0 dB. That way you assure at least that they all have the same peak level (not necessarily the same perceived loudness, see Edgar's post.) Then grab all the audio files at once and drop them into GB's timeline - they will all end up on separate tracks.