There are a very large number of YouTube videos on that subject.
i think that you could just put a 100W bulb a few inches from the eggs and wrap them in a cotton cloth. But double check with a guy at a pet store to be sure that you don't kill the thing.
Most of the times the people that work at a pet store don't know anything; each animal requires certain temps, so just sticking a 100w bulb over them is not going to work, and he most certainly does not need wrap them in cloth.
For what kind of reptile? Reptiles have unique needs when it comes to incubation and hatching of eggs, some more specifics would help.