The Amp Simulation editor will allow you to change the pre gain, low-mid-high levels, as well and the presence, output level and Master volume. (See Screen Shot) The model of the amp can also be changed in between American and British styles of amp sounds. 6 in all. Am. Clean, Am. Gain, Am. Bright gain and their British counterparts. Set up a new software instrument track with only a grand piano. Select this as your edit for that instrument and play with it. It will show you the range of manipulation that you have for amping the instrument.

Today's session will begin with a listing of the 17 editing instruments that come "stock" with Apple's Garageband.

Adding and Adjusting Effects

Each Real Instrument has a set of effects, which include a noise gate, compression, equalization (EQ), echo, and reverb. You can adjust a track's effects, and add up to four additional effects. 
Most of these are found in newer systems and more recent versions of GB:
Amp Simulation
Auto Wah
Automatic Filter
Bass Amp 
Bass Reduction
Bitcrusher
Chorus
Distortion
Flanger
Overdrive
Phaser
Speech Enhancer
Track Echo
Track Reverb
Treble Reduction
Tremolo
Vocal Transformer,
 all are listed in the Track Info window. 

I will go through each one seperately but the real way to learn it is to start GB and edit your own instruments!!!

 Within GarageBand, you can input music in three ways. One is pre-recorded loops. GarageBand is bundled with around 1,000 pre-recorded loops, with everything from guitars to percussion and brass. Second, you can input with any recording interface that's Mac compatible, from the built-in soundcard, USB microphones, or simple external interfaces. Third, you can use a MIDI keyboard to perform any one of the 50 included sampled and synth-based instruments. Expansion packs are available and very popular.

*Next: Let's take a look at how to create a simple song using GarageBand's included loops.