Here's how to do basic timestretching in GarageBand :
1-In iTunes, drag the song you want to import to the desktop.
2-Drag the song file from your desktop to a GarageBand. It is best to set the GB song to match the tempo of your song beforehand.
2-The new region will be orange.
3-Select the orange region and hit Ctrl-Option-G and click once just under the region. It will turn purple.
4-Double click the new Purple region.
5-In the Audio Editor, check Follow Tempo & Pitch.
6-The new region will follow the tempo of your project.
Keep in mind that other application like Melodyne and Ableton Live are better suited for this kind of task.