Start with the official practice CFAT on Forces.ca (Found under "How to Join > Steps to Join"). Use it as a guide on what to study, the questions on the CFAT will generally be of the same type and difficulty. Make sure you're ready to complete all of the problem solving questions (math) by hand within the time limit (as an aspiring engineer, this section shouldn't be too hard).
As far as being potentially employed at an embassy as defence staff, a degree in International Relations/Political Science has no bearing on such employment. Most officer positions at foreign embassies are for Lieutenant Colonels and Colonels, with some positions for Majors, and it is something that you apply for. Trade doesn't matter that much.
If you want to study International Relations and work at an embassy, you're looking at applying to the foreign service as a civilian (Global Affairs Canada).