Are you using the Kingmaker rules or the Ultimate Campaign rules?
I don't recall the internal Kingmaker rules, but here are the Ultimate Campaign rules
In general, the scenario goes something like this:
1) You may create, replenish, or expand a military unit during the edict phase of a kingdom-building turn. You can do this in place of creating a new settlement.
2) An army, from my reading of the rules, costs its consumption cost per week. In general, you have to pay this consumption cost up front. But if you have appropriate buildings, you can house your armies "in reserve" in a building, and only pay their consumption cost once per month.
3) A lot of army resources (mounts, healing potions, etc.) require that a kingdom have an appropriate building to supply the resource.
I think a lot of people also have house rules. Mine:
1) You must have appropriate buildings/resources to recruit army types. If you want to recruit paladins, there needs to be a temple or such to a LG god and an order of paladins in your kingdom. If you want to recruit an army of wizards, there needs to be a caster's tower or similar in your kingdom.
2) You also need appropriate people to recruit armies. If you want an army of centaurs, there need to be centaurs in your population.
3) Armies can't exceed more than a resonable (fuzzy percentage, defined by GM) proportion of your kingdom's population. If you have 500 people living in your kingdom, you can't recruit a 400-person army.
