Talk to your professors, especially if there is someone that you have done some research with either for your thesis or a course.
Ask them for advice.
Explain what you are interested in doing. The kind of research questions you find interesting. The kind of techniques you like to do. What your working and learning style is like. Ask them to recommend 2 or 3 professors that might be a good fit for you.
Tell them if there are any potential supervisors that you already have in mind and ask if they know that professor.
Professors have a lot of information about graduate school and are happy to talk with students. Talk to a few professors because they will have different experiences and knowledge about supervisors and programs.