Bruno, no disrespect but if you're asking these types of questions you will need help with your upgrade. The points I'd made are quite straight forward, and for sure there will be things that occur that are complex or surprising - it happens with all upgrades.
My strong recommendation would be to find a local Business Partner and get them to do the upgrade. As a new Max Admin you may want to work with them to get some pointers and experience.
To answer your specific questions :
1) Should i do the full backup by going through File=> quick backup
A : I wouldn't. I would copy the contents of the Address Book folder you're going to upgrade into another folder
2) do i do it twice the same way ?
A : I would. Otherwise you'd have to test each backup as one working is no guarantee another made using a different technique is working.
3) how do i create a new Max company? (just a normal folder?
A: Use File - New Address Book, pointing to the folder where the copy of the original Address Book is
4) how do i test the backup by retrieving it again?
A: Open the new Address Book created in #3 and see if you can read the data, if your reports work, if the record counts are the same, etc.
1- what type of notes should i usually take?
A: Detailed, but not overly so. Capture key decisions, key data, etc.
2-if i uninstall and reinstall first on my computer (before moving to the next 10 computers in the office) by retrieving the address book would al my administrator settings come back as well? or those are reset to default again? (because if they are are..i'll then have to take down notes of how they were before)
A: This is the kind of thing you test with your copy - along with all the other data you want to be sure is maintained. I don't know the answer as I haven't ever upgraded to Max 9.
3- would all the users (usernames and passwords) who had access to maximiser will have their's reset as well? i'll have to assign them again to them?
A: This is the kind of thing you test with your copy - along with all the other data you want to be sure is maintained. I don't know the answer as I haven't ever upgraded to Max 9.
- R.