The first step would be to look at what research is conducted at Mac - it's relatively easy to do too, because each Department's website has lists of their faculty members, and these usually include a little excerpt on what research each prof is doing, or even better, a link to their website. This might take some time but finding something that interests you really makes a difference, especially if you're doing it for the experience. Also, some profs are specific about what your application should be like/what they're looking for in undergrads/whether they actually even accept undergrad volunteers, and all that useful info is usually found on their websites.
Do not restrict yourself to one Department! Not only are you making it harder on yourself by restricting your options, you might be missing out on some really unique/interesting research too in other Departments (or Faculties). It's good to know what your likes and dislikes are, but being open-minded also helps
. When I was looking for positions, I looked at the Bio, Psych and Biochem departments.
When sending out emails, cater each one to the specific prof. I know this will take time and effort but the chances of them replying are better, from personal experience. Even then, be prepared to hear absolutely nothing from some profs :p Many of them are super busy and probably get lots of emails like this all the time (also why you want your email to be specific!).
I'm not too sure about the grades thing, I would say it depends - the prof whose lab I am in now just quickly scanned over my marks and instead talked to me about what my interests were. Other profs I ended up meeting though discussed a few of my courses and grades with me. It really depends, and I would say don't let your marks stop you from applying for positions.
The earlier you can apply, the better, I think. For my summer position, I started emailing profs around late Jan/Feb (which seems a bit ridiculous now that I think about it lol but it worked for me :p).
Hope this helps! Good luck in finding a position!