I have a 99 f150 4.2l when I try to start it I can hear the starter engage but won't turn over, I replaced the starter and took the belt off and tried it but still had the same results what next

Just out of curiosity, what belt?

Serpentine, was told maybe a pulley was froze

If you can hear the starter trying to engage after replacing the starter, I'd have to wonder if the battery had enough CCA to start the engine. Battery can read 12.5 volts, run the lights, stereo, but fail a load test. You either need a load test meter or a lot of auto parts store will test your battery

New battery, 900ca

Not many options I can think of right now. Years ago I got a replacement starter that was bad, but that was rare. There's a possibility that the neutral safety switch is malfuctioning but that's also fairly rare. http://www.ehow.com/list_7425651_signs-faulty-neutral-safety-switch.html