You don't need to press anything to talk. Your mic only "turns on" when you makes sound loud enough to pick it up. You'll know if your mic is picking up noise when you see a little icon on the bottom of your screen light up.
And the enemy team can hear you using "proximity voice". That means if you are near them, they can hear you, but if you are like across the map or something they won't. It's pretty much like hearing people talk around you in real life.
BUT, if you are in Xbox Live Party Chat, only the people in your party will hear you, so you won't have to worry about it.