Also, when you record, make sure your mic level is plenty loud, but make sure it doesn't clip.
You could also try using a program like Audacity to open up your stereo mix and adjust the volume in there.
Don't listen to this, it's wrong. Haha sorry, but that's not what you want to do at all. That introduces digital distortion and makes your songs sound much worse.
Like he said above, -18db, peaks around -9db.
I wouldn't raise the volume by moving the master fader either. I leave the master fader at 0 the entire time, unless I want to turn it down in my speakers. Generally adding gain at the fader is not a great idea, even though most sequencers can do it "alright" nowadays. A compressor is much better designed for the job.
Compressors are for raising the volume. Specifically limiters on the master, which is just a compressor with an infinite ratio. You've already exported the mix, which is good. Just import it again to a new project, or like I do, an new track in the same project. Mute all the other tracks, so you just have the mixdown. Then throw a limiter on it. That hopefully will be all you need. I suggest resisting using eq or anything other than a tinge of reverb on the master, since you're not a mastering engineer and don't really know what you're doing haha. Make the mix sound the way you want it to. It takes some time, but fight for it. Then like I said, the limiter, or compressor with a very high ration if Garage Band doesn't have a limiter.
If you're making an album of songs like he said above, you're more likely to need to use EQ so that all the songs have the same feel, sound-wise.
Edit: I believe Garage Band uses AU plugs, right? Here is a decent compressor
. Found it yesterday from a post on the Tweak forums, actually. If you turn the ratio all the way to 1000, it will essentially be a limiter so you can use it that way. You'll then have to play with the threshold (they call it 'sensitivity' on this one) and gain to get the sound good. Make the attack pretty quick, if not as fast as possible for limiting, and the release shouldn't be too long either.