A little unsure about the best way to use my military forces (brand new player with some noob questions)

So I've learned that I should be cranking out villagers. I now have a shitload of resources. Once I'm at this point do I just start sending waves of units at my enemy (while maybe keeping some back for defensive purposes?

Or is there a better way to do it?

I ask because the nonstop villager production and economy focus was counter-intuitive to me. I've been playing on easy/standard so obviously I can get away with not playing optimally, but prior to this I was just creating like, 15ish villagers and spreading them evenly across the resources. I had no problem crushing the puny AI armies this way, especially if I rushed them early and stunted their growth.

Now like i said I have plenty of resources (but I do end up with a bunch of idle villagers at times, especially when stone/gold spots run dry).

I also noticed that the AI spreads out quite a bit. Usually my first targets are the freaking buildings they throw up right outside my walls. I've been turtling and walling myself in...bad idea? Should I be more aggressive with my base building?