Already mentioned by
@Joey waid adding bottled nitrifying bacteria Would be good. Can tell you that Dr. Tims will probably win here in votes, but if your going to a Petco and they dont have Dr. Tims there is ATM's.
Here is a diagram of whats happening. When you add food or when your fish poop, ammonia is created. Bacteria comes in eats the ammonia and leaves nitrite, another bacteria eats the nitrites and leaves nitrates. Nitrates are removed through WC's, Bacteria, plants/algaes, and corals. For the cycle to continuously flow in a healthy balance the bacteria needs a strong foothold. This is done in various ways. Popular methods are with live rock, live sand, live water, time, bottled bacteria, or a piece of raw shrimp. My gut thinks the best method to start cycle is with live rock, live sand, and time. If done right you can completely skip creating a dangerous level of ammonia from the get go. This is just what I do or would do.
My new tank is going to start cycle with live sand, dry rock, 48oz of ATM colony, and a pair of QT'd clownfish. Ive never tried starting with bottled bacteria before