The hot water to my bathtub and shower is really hard to turn on and off hoe do I fix it the stemsn are new

Did you put any kind of lubricant on the stems before you installed them. If they have o-rings they need some lube to make them easy to turn and to not tear the o-ring. If they have a packing nut it is possible that the packing nut is too tight.

Good advice by LetmetellU,

Before you remove the stem to lube the threads and "O" rings go for the bonnet packing nut,(see image) first. I think you overtightened it when you installed the stem. Remove the knob a chrome cover. Now slip the knob back on and open up the valve so the water starts to come out. Now back the bonnet packing nut off counterclockwise until water begins to seep out past the stem. Now slowly turn the packing nut clockwise until the water stops. You have just adjusted the bonnet packing nut. Does it turn easier now?
Cheers, Tom