First of all, all the supervisors of graduation thesis will give the topic of the thesis for everyone to choose. Generally, there are more topics and directions than the number of people writing papers, and then everyone chooses topics separately. If two people choose the same topic, then negotiate and let one of them change the topic. This unwritten thing won't have much impact.

Then start writing papers according to the requirements of the topic. If you don't understand anything, go to the information and ask the tutor for help.

Mathematics has many directions. Choose a project that you think you can learn and go further. Start (a disease)