Before you export from Garageband check to see the levels in the Master track. It should be as high as you can go without hitting the red.
Alternatively, you can use a Limiter plugin to increase the loudness of the track.
This process you're talking about is called "Mastering". It's the final stage of the music production process and is concerned with polishing the final mix and making it louder (without adding distortion or clipping the signal).
You can find plugins for mastering (e.g look for iZotope's Ozone) and applications like Logic Pro come with mastering tools to help make better masters of your music.
Hope this helps
Rounik
P.S. Could I ask yo to add a name that will identify you on the forums - otherwise I won't be able to be notified of any of your replies to this post. Click on the "Forum Settings" link. Thanks!
Choose Track > Show Master Track and the Master track will appear at the bottom of the track list in the main area.
You can automate it's volume level... but be careful that the level meter (in the bottom tool bar) doesn't ever go into the red. This will result in clipping and distortion of the audio signal.
You can also try adding an effect on your Master track. Try using the included AUPeak Limiter to boost the levels. Again, listen carefully to the changes you make so that you don't end up boosting the volume too much and therefore creating a distorted or clipped audio signal.