Launch GarageBand and create a new project at the splash screen on start-up. Otherwise, select "Open Existing Project" in the lower right hand corner of the window.
Create a new track by pressing "Alt-Command" and "N." In the pop up window, select "Real Instrument" and click "OK."
Record your vocal part, or drag and drop an audio file onto the arrange window next to the newly created track.
Click the "i" button at the lower right corner of the arrange window to open the track properties window. The top half of the window will display the available effect presets. The lower half shows the track properties, allowing you to fine tune the parameters of the presets, or create your own from scratch.
Click on the "Vocals" category on the left of the presets window. This will open the expanded list of available Vocal presets. Double-click "Radio Effect" from the bottom of the list. You can adjust the specific settings of the preset by using the effect sliders in the lower half of the window.