Cause of motion

Aristotle says that "everything that is in motion must be moved by something" (241b). This question is mostly a point of clarification. Does that allow for something to be moved by itself? Can something contain within itself the principle necessary for motion? Does Aristotle mean that everything that is in motion must be moved by something else? If he does, how does the idea of a first movent fit into this? Is the first movent itself in motion?

It seems to me that the first movent would have to be at rest because if the first movent itself is in motion, then it would need something to put it into motion. However, it is the first movent, and therefore, it does not have anything to put it into motion. On the other hand, I don't see how something can put something else into motion while being at rest, because putting something else into motion is a process of motion itself.