Heli won't lift off the ground!

Okay, here's the deal. I just got my belt-cp. I haven't flown (or attempted to) it yet, and have just recently figured out all the controls on the transmitter. I now know what controls what, especially the knobs on the top of the transmitter. Now, here comes the problem. I turned on the receiver, connected the battery, and powered up the heli to check all the mechanicalstuff. Servo's work good, channels from transmitter work good. I throttled up (still had thottle lever turned down), RPM picked up, main blade speed picked up, throttled all the way up, and heli never got off the ground. I was testing on a concrete floor, and all it did was skid across the ground. What could the problem be? I had the knobs on the top of the transmitter set a "0", should I adjust the trim of the main blades? Again, anybody who knows what to do...HELP!!!