Open Garageband and start a new project. At the prompt, click "Start a New Project." Name your project and save.
Set a tempo, time and key. Garageband will automatically prompt you upon beginning a new song. All of these decisions can be modified later, but the program encourages you to begin with a basic sense of how you want your song to play out.
Add a track. Go to "Edit," "Add New Track," which will prompt you to select an instrument. Once you have chosen your instrument, the track appears on the timeline.
Press "Record" and begin playing your instrument. The track will automatically progress, with a metronome to indicate the speed set when you first began the song. No matter what instrument, the standard input is the keyboard you will see on screen in front of you. Playing notes on this keyboard while pressing record will lay those notes down on the track.
Click the eye to add pre-recorded loops. Useful for making beats, pre-recorded loops allow you to drag-and-drop specific recordings. You can drop multiple copies of the same loop on the same track to keep a through line going within your piece.
Export your track to iTunes. Once you have laid down multiple snippets of recording on several tracks and adjusted the arrangement to your liking, export the full track in MP3 format for use.