A substantive thesis says how things are. Therefore, state your thesis briefly and clearly near the beginning. Sometimes your thesis will not be fully understandable until the argument proceeds a bit. If so, alert the reader and offer a crude approximation, identified as such, and clean it up later. Typically, one doesn't know precisely what one's position is until one has worked out the argumentation.
Many students write until they arrive at their position, and stop.
Proper and careful formulation is three-fourths of the battle. Make your sentences say just what you want them to say. If you feel dissatisfied with a particular sentence, chances are good that you don't yet know what, exactly, you want to say. Write as if your readers are going to be members of the class who missed the relevant material. Don't write for the prof.