Any courses at the 300+ level can be used toward the 18 credits required for the BSc minor except for a few whose descriptions in the Calendar rule them out (e.g. MATH 335). MATH 200, 215 and 221 are the most common prerequisites for 300+ courses so it is almost certain that you will have to take some of these as well. These courses will also help you get a better sense of your interests. In general, talking to your peers, particularly those a year or two ahead of you, is a good way of finding out about courses that may interest you.