Well be prepared for a lot of headaches, dead ends and frustrations. Greeks quite simply aren't very good at dealing with "official" type things and get confused very easily. You'll need a lot of patience, some creativity and a supply of liquer doesn't hurt either. I can only speak for how to obtain documents in America as I have zero knowledge of how things work in Canada.
1) This is a document that does not exist in America (you'll run across this a lot - europe has many more layers of bueacracy than America). Basicly you need your high school registrar to draft a letter saying you had satisfactory grades in your classes. This is redundent - I know. The more stamps and seals that they can put on this the better - Greeks are easily fooled by such things.
2) Another document that doesn't exist in America. You'll need a letter from someone (i.e school registrar, principle etc.) stating you would be eligible to enter the field of studies you are entering in Greece in America. Make this as official looking as possible.
3) This requires a few things actually. You need to prove that you are allowed to enter a university in Greece. I didn't personally have to deal with this document - but for non - greek residents I know part of this is proving competence in Demotic Greek.
4) You need a statement documenting your ethnicity (yeah I know this is strange). In my case it was simple - my parents simply wrote a letter stating they were Americans and not of Greek descent, got it notarized and that was all I needed. I think the procedure is more complicated if you are of Greek descent.
Oh and don't expect anything from the consulate. And if you get frustrated by the consulate, keep in mind that the VERY BEST Greece has to offer staffs their consulates in America, Australia and Canada.
I was also told that if you have greek descent you need to take entrance exams. is that true? even if you are from North America?
Depends on what percentage of Greek you are, when your family left Greece etc. I don't remember the numbers off hand since being 0% Greek it was something I didn't have to deal with.