The Outfit Workshop unlocks when you have 32 Vault Dwellers and costs 1200 Caps. You can place up to 6 Dwellers in each crafting room, and more people, the less time crafting will take. You need at least one Dweller in the workshop to begin crafting.
You will have a good amount of crafting recipes, while unlocking more through exploration, raiders and lunchboxes will yield better equipment to create. You can also use Quantum to speed up the process.
Each recipe will call for Caps and a combination of Junk items that you will likely have a stockpile of at this point in the game. You can also scrap Weapons and Outfits for a chance at more scrap.