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Click Track. Click New Track. Click Software Instrument. Click Create. Select an instrument. Bring up the Musical Typing window. Click the "Record" button. Play your instrument. Stop the recording. Loop a recorded instrument. Split a track. Add and record more tracks.
It's a menu item at the top of the screen. Clicking it prompts a drop-down menu. You'll find this option near the top of the drop-down menu. It's on the left side of the pop-up window. This option is at the bottom of the pop-up window. Doing so adds a new track to your GarageBand project. In the "Library" section on the left side of the window, select a category of instrument, then click the specific instrument you want to use for your new track. You can first edit your track's preferences by clicking the knob icon on the right side of the track's box and then changing any necessary settings in the pop-up window. Click Window, then click Show Musical Typing. This will ensure that you have a reference while recording your music. It's a red circle at the top of the window. Once the four metronome clicks play, you can begin playing your instrument by pressing the keyboard keys which pertain to the notes you want to play. Click the "Record" button again to do so. This will save your track. Click and drag the upper-right corner of a recorded track to extend it into a loop. If you want to divide a track into two separate pieces which can be moved independently of one another, drag the playhead to the point at which you want to split the clip, then press Command+T. Once you've added the main track for your music, you can add additional tracks with different instruments (e.g., bass or synths).