The criteria for joining the P.Res can be found here:
http://www.army.forces.gc.ca/land-terre/joining-enroler/rfep-pefr-eng.asp (Enjoy the links on the left hand side)
As per joining, you will have to pick a P.Res unit in your area and initiate a file with them. This usually consists of a meet & greet and inquiring on whether or not there are any positions open. If there is, you will be given a letter of recommendation which in turn you will bring to your local CFRC to begin your paper work.
Units in your area: http://www.army.forces.gc.ca/land-terre/units-unites/index-eng.asp
CFRC: http://www.army.forces.gc.ca/land-terre/joining-enroler/eligibility-admissibilite-eng.asp <--- Ensure you have the supporting documentation if you want to speed up your process a little.
Per meeting up with "the group", most reserve units have PAT pl's that parade with the qualified pers one night a week, and with the possibility of 1-2 field ex's a month. Inquire with your chosen unit's recruiter.
Joining the P.Res while in high school isn't a bad idea, it may give you a taste of what the real world is without your Saturday morning cartoons
Regardless, stay in high school and if you can, attend a post-secondary school before you make your decision to go Reg.F...
Good Luck,