At level 90 (or with your 120 if you boosted) you can fly to the Dark Portal and Archmage Khadgar will take you on a pretty exciting, lore rich and quite long quest chain that eventually sees you set up a garrison.
I'm going to pick up the strategy from there and talk about what you need to do to build an effective and powerful garrison that can add to your income.
Let's review the buildings in a tier list:
Tier 1 (powerful).
Town hall - quest hub, resource cache and prereq for everything else. It's best to upgrade this as soon as possible. If you don't have enough of the garrison resources character you can simply do other stuff for some days or weeks while your cache slowly fills or you can do some quests that reward garrison resources. Empty the cache every 3 days or so minimum.
War mill. The main source of follower equipment, needed to unlock the best missions. The transmog gear can be disenchanted for mats. With a level 3 war mill you can turn in a few scraps for more follower equipment upgrade tokens.
Frostwall tavern. Main source of new followers. Improves garrison resource missions.
Trading post. This is the sink for garrison resources and necessary to make gold. You only need leve; 1.
Tailoring Emporium. Allows you to make Hexweave bags even if you're not a Tailor. Curently the best way to turn excess Garrison resources into something sellable.
Salvage yard. Passive garrison resources.
Tier 2 (useful)
Barracks - allows high experience patrol quests (perhaps redundant once you've levelled your followers), allows a bodyguard (pointless) and increases follower cap by 5 (highly useful).
Alchemy lab, all other profession buildings. Turns resources into specialised resources, sells recipes, some minor perks. Useful if you have that profession.
Storehouse. Improves throughput of work orders. Bank and guild bank.
Tier 3 (rubbish)
Barn. A convuluted way of getting skins. Easier to farm garrison resources and buy them from the Trading Post.
Spirit Lodge. Moving around Draenor isn't important these days.
Stables. Good for completionist mount collectors and that's about it.
Gladiator's Sanctum. There's no one to pvp on Draenor.
Lumber Mill. A mediocre way of getting extra garrison resources.