I want to record myself playing guitar in iMovie while running garage band for the real time sound

Whenever I open garage band and set my instrument and track

To play over I then open IMovie. When IMovie is open

It does not have a picture from my camera on my lap top, it's blank.

The same when I open QuickTime. When I open iMovie or QuickTime first,

I get a picture, but when I open garage band next it does not produce any sound

And I have everything set to use the USB interface as the audio source.

Help! How can I record myself playing over a track in real time?