While he did not "steal some other guy's work", nothing in science exists apart from what has come before. Einstein's insights were aided by centuries of previous investigations onto the nature of matter, and of energy. His particular genius (and that is what it was), was his ability to set up novel "gedankenexperiment", or thought experiments in his head where he could imagine how the world would have to look from different points of view. This directly led to Special Relativity, as he saw that there must be an absolute speed limit for the propagation of information, which in turn led to his realization that space and time are linked in a relative way -- that where and when you observe something depend on your velocity to the observed system.

Please have a look at the PBS Nova episode "Einstein's Big Idea" for a wonderfully detailed, accessible and produced telling of the tale. Key in the story is his relation with his first wife, Milena, a mathematician who no doubt made important contributions, directly and indirectly, to his early work. Sadly, their marriage was not a happy one, as he was emotionally distant and tied up in his work. He got her to agree to a divorce by promising her that when he won the Nobel Prize, he would give her the money. A bit of a ballsy bet on both their parts, but it does show how much faith they had in his work