If I know my average grade for each of my 2 years at Mac, how do I find out the cumulative average? is it as simple as adding them together and dividing by 2, or is it something else?

You have to convert each individual mark to the 4point scale if that's what you're trying to calculate. If you just want it on the 12-point scale then just divide the sum of all your marks by 20 (or however many courses you've taken so far)

CA= sum(grade * #of units)/total units

If you've only taken 3-unit courses then you can do a normal average, but if not then it's weighted based on number of units.

For first year, ur grade report should say for example " CUMULATIVE AVERAGE: 9.1/070 UNITS". This means u have an avg on 9.1*70 = 637 point over 70 units.

Now using the eqn Starfish mentioned, find how much u have overall including first year.

Example:
(Below are 2nd year marx)
CHEM 2A03: 5
PHYS 2C03: 6
MUSIC 2L04: 9

CA = [ ( 5*3 + 6*3 + 4*9) + 637] / [ (3 + 3 + 4) + 70 ] = 8.825

hope this helps

how do fail marks count in? and if i retook a course?

All of it's just added in. So a 0 would mean you add a 0 to your average calculation and then divide by that extra input you used. ie. (6 + 6 + 0)/3 = 4 average GPA

Similarly with retaken courses (depending on what you're using your average calculation for), but for McMaster the average takes the retaken course the same as any other course you decide to take (it's as if that course is just another course, and does NOT replace your previous mark for that course).