V2.1.1 for iphone basic question

I just purchased Garage Band simply to record a guitar track, then later record a vocal track with (over) the previous guitar track. I have a 1/4">1/8" jack so I can plug my guitar directly into the phone. I figured out how to record the guitar, but am having trouble getting the vocal track. I need to (obviously) listen to track 1 (guitar) while I lay down track 2 (vocal). How do I do this? I do not have a mic so I'll be relying on the phones mic. I DO have, for my computer, headphones with a mic...but this is USB...if I could find a USB to 1/8" jack would I be able to listen and sing through the same phone jack? This is option 2 if available..option one being plug ear buds into the phone to hear the guitar and sing into the phone mic for track 2.

Thank you in advance for the help!