Open the GarageBand program on your computer. Open an existing session.
Select a track in the session window. Move the cursor over the track you wish to add an effect to and highlight it by clicking the mouse on it once. For instance, if you want to add an effect to a guitar track, click the guitar track in the session window.
Open the "Track Info" pane. In the GarageBand menu at the top of the screen, select "Track," then select "Show Track Info."
Select "Real Instrument" at the top of the "Track Info" pane.
Click on an item from the menu on the left. For instance, select "Guitars" if you want to add a guitar-related effect to your guitar track.
Select your desired effect from the list of items on the right side of the "Track Info" pane. This automatically adds an effect to the entire highlighted track.
Open the "Details" tab at the bottom of the "Track Info" pane to further refine the selected effect. Here, you can fine tune the effect, as well as add echo and reverb to the track, as desired.