It’s very helpful to think of the development of an essay in stages. Compared to building a house, you need to decide what materials you will need to build this house, you need to go out and gather those materials together, you need a plan for putting them together so they’ll be what you set out to build in the first place, and then you go over it to make sure it’s complete and doesn’t need more materials and work or even, perhaps, need something removed that wasn’t really necessary after all.
You must first settle on a topic—a broad range of information. Then you must find what materials there are about that topic and then you being selecting, rejecting, and looking for more. This is what is called discovery. From the discovery stage will come the thesis—the point you want to make about this topic. You will probably write more than one and will need to discard and hone until the thesis says exactly what you want to say. It’s usually good to keep track of where materials came from and to organize what you have found by areas of concentration.