Depends upon what kind of lizard. An insect-eating one wouldn't get far on lettuce!
To be honest, if you don't know already, perhaps you shouldn't be keeping lizards at all...? Reptiles need you to get their conditions exactly right, including food, otherwise they die.
Go to the shop you got it from, and ask them.
However, if this is a baby you have bred from adults, then you must be doing something right... In that case, give it similar food to the adults, but smaller and softer.