Advice for new player

Hey everyone, I recently just picked up AOE2 DE and I have a question about defense. By nature I’m a very reactive person so I try react to what my opponents build but often times I get blindsided by a scout rush when I see them producing infantry and other things like that. I just want to know what’s the most effective way to go about producing my first military units of the game. Do I just pick a unit (like archers or infantry) to produce until i see the opponent building something and then adjust to counter?