GarageBand doesn't have an upload to YouTube option. I suggest you email the song to yourself from GarageBand, or "Send to iTunes" in AAC format. Click the arrow tool at the left below your songs when in the My Songs view.
Once you've got a copy of the song on your computer either from the email, or by copying it out of the File Sharing area for the GarageBand app, you can import it into your iTunes library, sync it to your iPad and then use it as the audio for an iMovie project. At the very least I would think that you would want a static image and some titles to make a video that you should then be able to upload to YouTube directly from the iMovie app.
tt2