Welcome to the Mac.
The answer to your question is quite simple.
Click on the track that you want to edit and you will see 5 little icons under the name of the instrument.
The right hand icon, a triangle pointing downwards, is the one you want. Click on it and you will see 2 options. Track Volume and Track Scan. You want Track Volume.
Click on the line at the point at which you want the track to start fading. Now click on the end of the track and drag it down. You should have a slope starting from your first point and going to the end.
Guess what that does to your volume?
Hope that helps
Ok, so I am a big newbie when it comes to handling Mac applications.
In Garage Band, how do I edit a track so that the sound fades at the end?