MSC (scroll-able 2010 version)
This is the list of Mathematical Subject Classification numbers from 2010 - it is missing some new subfields compared to 2020 - BUT it is extremely detailed and may help you narrow down some interests you might have. I would start with the main numbers first (such as 92 - Math Biology) and then look at more specifics depending on what fields you are interested in (such as 92C60). Having a variety of ideas and interests will help you later on down the road!
Click on MSC (scroll-able 2010 version) to open the resource.