I'd probably end up a rebel.

Apprentice G-Man ;)

Cascade resonancely.  
 
Or I wish I could grow the balls to rebel, but I'll probably just end up living in City 17. 

I'd be a sheep, living out my days in city XX. (No not literally a sheep...)

Honestly? Probably? 
 
I'd shit my pants and wait patiently for the end. 

I'd either start fucking running and hide or join the rebels.

All hail Our Benefactors.

Whoa now, the resonance cascade, the Combine invasion and the appearances of the G Man are all very different events. Also, I believe the Combine simply enslave the human race instead of allowing them into the military and there's no way anyone would be able to choose to get involved with the G Man, it seems like you don't find him, he finds you. In the event of resonance cascade I'd keep on living my life and be glad I live in the UK, however in the event of Combine invasion I probably would just become one of the enslaved many.