Hi! Great to see you're looking into McMaster for social work. They typically admit 45 people into the BA/BSW program and 25 people into the post-degree BSW program. The year I was admitted we had over 250 applicants. The admission weight is 50% grades, 50% SWAT. You need a full credit of intro social work or sociology, and then another full credit of more social sciences (psychology, anthropology, women's studies, etc, can be two half credit courses). The minimum average required is a C+, although aiming for a B+ is probably best for you.
The SWAT essentially tests your personal suitability for McMaster's program. You'll be asked to give your perspective on a social issue, then discuss why you would like to be a social worker (here you can highlight any experience you have, personal or professional), the last part is a case study where you have to discuss how you would help the individual. The best way to really prepare is to try and view the world through the philosophy of the School, which can be found here:
http://www.socialwork.mcmast er.ca/
best of luck!