There are two ways to recruit normal units:
1) Build them yourself.
2) Pay mercenaries to join you.
The first option requires more advanced buildings for more advanced units. For example, halberdiers require an armory and bowmen require a sawmill. There is also the time spent building the unit, rather than adding another building; each unit also has a cost in both food and gold.
The second option is dependent upon your fame: the more you have, the more often mercenaries stop by. There's a formula, but in general low fame equals one weak unit occasionally; high fame equals three strong ones fairly often. Having the Famous retort doubles your chances per turn, while Charismatic halves the cost of hiring whatever mercenaries do appear. (You still have to pay their normal food and gold costs, however.)
One last thing: mercenaries only approach your wizard's fortress. Which means that if you want to use them elsewhere, it's up to you to arrange transport (by land, sea or spell).
If this answered your question, please be sure to take a moment and mark it as the solution. If not, then let me know what I've missed (or misinterpreted).
Post edited October 03, 2013 by TwoHandedSword