It can be quite a challenge for a student to decide what topic to choose for their first documentary film. My advice is to start with a topic that you are passionate about. Think of something that you are interested in and you would love to explore on a deeper level. Make a list of topics and then you can start researching and brainstorming for ideas. 

The next step is to talk to people who have experience and knowledge of the subject. Listen to what they have to say.

You can also watch existing documentaries to gain inspiration and figure out how to approach your project. Good luck!