Connect any instruments to your iMac. If you just want to record through your iMac's internal microphone, skip to the next step. If you want to connect a microphone, guitar, bass or other instrument directly to your iMac for cleaner recording quality, purchase a USB instrument cable. Sold in music-supply stores, these cables allow you to record seamlessly through your USB port. For most analog instruments, use a USB-to-TRS cable. For XLR microphones with a three-pin connection, connect an XLR-to-USB cable.
Open your System Preferences. You can access them by clicking "Apple" then "System Preferences" on your main menu bar, or by clicking the “System Preferences” icon on your Apple dock (along the bottom or left side of your desktop).
Click the “Sound” preference and then click the “Input” tab. You will then find a list of input options. If you plan to record using your internal microphone, select “Internal Microphone” from the list. If you connected a device through your iMac's “Line-In” port (such as a 1/4-inch-to-1/8-inch guitar cable), select “Line-In” from the list. Click the red button in the upper-right corner of the window to exit. If you connected a USB device, proceed to the next step.
Open GarageBand. In the upper-left corner of your screen, click the “GarageBand” menu and select “Preferences” from the drop-down menu. When the pop-up window appears, select the “Audio/MIDI” tab.
Click the “Audio Input” bubble. If using your internal microphone to record, select “System Setting” or “Built-In Audio” (depending on your version of GarageBand) from the list. If using a USB device, select the name of the USB device from the list. Click the red button in the upper-left corner to exit the "Preferences" window. You can now begin recording.