Create a new GarageBand project by opening GarageBand, clicking "File" on the menu bar and selecting "New" from the drop-down menu.
Click the "Create New Music Project" button.
Enter a title for your beat on the "New Project" window. Just beneath it you can choose your key and tempo. For slow beats, try choosing a tempo between 90 and 105 beats per minute. For fast beats, try something in the range of 130 and 160 beats per minute. Click "Create" when you finish entering your information.
Create an audio track by clicking the "+" button in the lower-left region of your GarageBand window and selecting "Real Instrument Track" on the pop-up window. Your new track will appear as a long, horizontal workspace near the top of your browser.
Place a percussion track on your first track by clicking the "Loop Browser" icon in the lower-left region of your GarageBand browser and clicking the "All Drums" button. Your loop browser icon looks like an eye.
Choose a drum sound from the list at the bottom of your GarageBand window. Click a loop once to hear what it sounds like, and use your mouse to drag your favorite drum beat directly onto your blank track. Make sure to place it at the very beginning of the track.
Repeat your loop. You can stretch your loop across the time line so that it repeats as many times as needed. Just hover your mouse over the right edge of the loop until your cursor becomes a circular arrow, and then click your mouse and drag the loop to the right.
Layer additional tracks. You can add additional drum loops for extra percussion, and explore the bass lines, sound effects, synths and additional samples--all of which you will see on your loop browser. To layer your different sounds, create additional tracks and place each loop on a different track, so that it sits directly beneath the loops that came before it. Mix and match a variety of different tracks to create your own unique beats.
Mix your tracks. Each GarageBand track features a volume fader and a pan dial. With the click of your mouse, you can raise and lower the volume of each part of your beat as well as moving different tracks between the left and right channels using the pan dial. For example, if you have a snare drum loop that you think would sound best in the left speaker, you can just turn the dial to the left on that particular track.
Add effects. Click the "Track Info" button in the lower-right region of your GarageBand window, and then click "Details" at the very bottom of the column. You will then see a list of effects options, allowing you to add reverb, echo, compression, EQ (equalizer) and a variety of filters. Select any track you would like to edit and click the various effects to apply them.
Save your completed beat as a song file. Click "Share" on the menu bar and select "Send Song to iTunes" from the drop-down menu. Select "MP3" as your format.