Choosing a supervisor is one of the most important decisions you'll make in graduate school. They'll be your mentor and adviser, and you'll work together closely throughout your graduate career.
You should select a supervisor with a strong record of research and publication in your area of interest. When looking into potential supervisors for your program, become familiar with the type of research they do and their working style.
Contact each of your potential supervisors to discuss your research interests, and see if they would be interested and available. Be sure to ask them about their availability during your graduate program, and whether they're planning any extended absences during that time.
When considering potential supervisors you should talk to several professors before asking one to become your supervisor. Be sure you understand what your prospective supervisor would expect of you as a graduate student.
The department's graduate administrator will be familiar with faculty members' expertise, teaching and research schedules. They'll know how many graduates each person is currently supervising, and can often help you create an initial short list of likely candidates.