Which hand shall I use to shoot?

You need to figure out if you are left or right eye dominant. Once you figure that out, adjust accordingly and shoot with your dominant eye. To figure that out hold your thumb out in front of you and center it on an object. Then close your left eye, keeping the right open, then repeat doing it opposite. Your dominant eye will stay on target and your weak eye will appear to shift (for me the left eye shifts off to the right).

it more depends on which eye is the dominate eye. that is the determining factor

I'm left handed but I usually shoot right handed. I can shoot both fairly easily but since I am right eye dominent and most weapons are right handed, its easier for me. Just practice which ever would work better for you, although it never hurts to practice shooting off hand incase your shooting arm becomes injured.

use them both if your ambidex' it will come in handy to be able to shoot well both ways you will never have to worry about position to get a shot off!

if your talking about going into the Armed Forces, you don't have much of a choice, although there is some choice, but most automatic rifles, after firing when they are reloading the bit that reloads it will fire back and break your jaw, so use your right hand

Since most rifles are set up for right handers you save
yourself custom expenses and not having empty cartridges
flying in front of your face by going right, if as the man says
your dominant eye permits. For pistols I recently started
training with both hands, and it's really fun. Now I can
fire with both hands like a hero in a Jon Woo film. ; )