The empty needle. You'll actually be knitting with FOUR needles. It's going to feel awkward and like you don't have enough hands, and that is perfectly normal. Bring those two ends together, working yarn coming from the right side and using the empty needle you start knitting those stitches from the left needle. Don't worry too much about that first join. It'll close up better with the second round.
One thing to remember though when it all sounds confusing is that even though you have so many needles you really are just using two needles like you always do.