but mainly how do i start with them?and build them up some
The most straightforward way to start is to begin creating cities that produce military units and nothing else. My rule of thumb is that you should have at least a quarter of your cities should be dedicated to producing military.
On the outside, a military city in the BC years usually looks like 1 or 2 food specials, and a bunch of hills (preferably green). The food specials are given whatever the appropriate improvement is to hook them up, and the hills are mined. You always work the food specials ("Always work your best tiles"), and spend as many turns working the hills as you can.
For instance, you might have irrigated corn (which you farm), four green hills (which you mine) and one brown hill (which you mine). At size 6, this city produces 17 hammers per turn, which is about half an axeman per turn.
On the inside, a military city... well, it looks kind of barren actually. You probably have a barracks. You might have a granary. You might have a monument. You might have a Stable.
But Libraries, Monasteries, Temples? Markets? Theaters? Those aren't UNITS. You don't build them here. Instead, you just keep cranking out more units. Yes, that deliberately avoids building happiness and health buildings, so your city isn't getting bigger. That's fine, getting bigger isn't units either.
Meanwhile, your other cities don't build any military (because your military cities produce plenty), so you don't bother building a barracks or a stable either. In other words, instead of having each city contribute 25% of your military, you have the military cities do all of it and the regular cities do none.
So first try just that much.
The next variation is a bit harder, but is probably key. Try to dedicate a quarter of your cities to commerce. What this usually means is a city that has the job of working as many cottage tiles as it can. Because it is working cottages, it isn't working mines - you don't have a lot of steady production in these cities (not a lot: 10 hammers per turn or less. More likely 5 hammers per turn or less).
At this point, you have half of your cities generating military, and half are still hybrids. Usually, the remaining cities fall into two piles - your really good cities, and non-descript remainder. So the good cities get interesting national wonders, and the non-descript cities provide support however they can.
For example, it's common to try to make an uber commercial center out of the capital, to take advantage of bureaucracy. But an uber commercial center also wants an Academy (because most of the commerce is being converted to research in this example). So you want an Academy - so some other city get a library and runs scientists to produce the academy you need. Need to spread a religion? then somebody pumps missionaries. You'll need workers, so somebody has to do that.
. Oh and I think courthouses can be a good thing if the empire is getting a bit large but NOTHING else.
