1500 Z71 GMC 5.7 engine my lights come on in my truck but when I turn the ignition to start my engine won't turn motor over at all

First would be to check the the battery to see if it is fully charged. Check the battery connections, clean the battery connections and inspect the battery cables for corrosion. If all this is good, then you would need to check to see if power is getting to the small wire on the starter solenoid when the key is turned to the start position. If not then it could be the ignition switch, or the neutral safety switch. If power is getting to the small wire on the starter solenoid, then there is a starter problem, and the starter would need to be checked.

is it auto trans or manual? there is a neutral safety switch that goes bad also if it is a auto tranny the switch on the transmission could be bad? check those out before you do anything else and like dude said check battery. batteries are only good for 3 years, i do not care what anyone says! they are the biggest problems of electrical problems