There are numerous jobs available within the reserves a 17 year old could get within the Combat Arms, Combat Support and Combat Service Support. Plus Navy and Air Reserves(if in your area).
The best thing you can do, is to figure out what Reserve units are in your area and find out what they do and what interests you. You can visit the unit, and search for the units and trades online for information. As well, by contacting the unit you can find out what they have openings for.
To find out what units are in your area, you can do a google search, example
"Army Reserves in Toronto"
"Navy Reserves in Vancouver"