Obviously you know where the Go Station is (on Steeles) so just go there and you're looking for 1 of 2 buses, both of them will bring you to McMaster.
You want either the 47 or 47A.
The 47 is just the regular bus so it makes a bunch of stops on the way to McMaster. It stops at a bunch of carpool lots along the way to pick up students. Due to this it takes longer to get to the school.
The 47A is the express bus. It takes the 407 and only stops at Square One before going on to McMaster. It's a shorter trip. Depending on traffic I'd say it's an average trip of 50 minutes, sometimes less, sometimes a bit more.
This is the schedule for this route:
http://www.gotransit.com/publicroot/...&station=&new=
I just went to Schedules > Train and Bus > Choose from a list > and pick 46, it's purple.
Look along the top of the schedule for 47 or 47A, then look along the side to see the times they arrive at the Bramalea Go Station and the time they will arrive at McMaster.
Once you make up your class schedule you can look at the Go Bus schedule and find the times that you will need to get on the bus to get to school in time for your classes.
Hope that helps.
If you're going to be commuting every day you'll want to purchase a monthly pass. The price was mentioned earlier in this thread, it is $226 for a monthly pass between Bramalea Go and Mac. You get unlimited monthly rides with that pass.