Open your current GarageBand project. You can find the GarageBand software icon in your "Applications" folder (Macintosh HD > Applications). If your project does not load when GarageBand opens, click (File > Open) on the menu bar and choose your project using the navigation, or just select (File > New) to create a new project.
Create a new track. The tracks are the horizontal bars that take up the majority of your project window, and they represent layers of music (like vocals, guitar, keyboard, percussion). To create a new track, click the "+" button at the bottom of your project window. A pop-up window will ask you to choose a track type. Select "Real Instrument" and click "OK."
Locate the beat that you want to import, and drag it from your hard drive directly onto your new track. As long as the beat file exists in an Apple-approved format (such as MP3, AIFF, M4A and WAV), it will automatically import into your project. After it imports, it will look like a long, colored horizontal bar with wavy lines (indicating the audio wave forms). To move it to a specific spot on the time line, just drag it left or right with your mouse, while observing the time line along the top of your project window.
Import Apple loops into your beat. In addition to importing external files, Apple's GarageBand contains a built-in "Loop Library," which contains hundreds of audio samples that you can import into any song file. To access the library, click the eye symbol at the bottom of your project window. Your project window will expand with a list of loop types (like "Drums" and "Bass"). Click any loop title to hear what it sounds like, and drag a loop onto a blank track (or blank area of your screen) to import it into your GarageBand project.