If writing a thesis, you must be good at one aspect of astronomy, then start with the one you are good at.

For example, I have a lot of knowledge about black holes, and I can start with them.

The definition of a black hole? This can be searched from books or encyclopedias you have studied. Moreover, it has great expansibility, even from the initial concept of black hole to the current theoretical system of black hole.

Conditions for the formation of black holes? This can be introduced from the most basic concepts in physics.

What are the properties of black holes? Why is it called a black hole? What are its special properties?

What is the significance of the formation of black hole system? To expand, we can describe the relationship between black holes and the study of the universe.

You must give your opinion. For example, what do you think is in a black hole? The last point needs more time, it is difficult to put forward your own ideas, and it must have scientific basis.

From the above, I have written dozens or even hundreds of papers. If you choose astronomy, there must be something you are interested in.

hope this helps