You'll be able to unlock this really early on during the campaign. Some basic info:
- You'll need 150 ethanol for your first infrastructure upgrade in Prosperity.
- You get ethanol by liberating outposts (+extra for no alarms/undetected), scrapping and re-liberating outposts, intercepting supply drops, and hijacking ethanol tanker trucks (you must drive these back to a liberated outpost to collect).
- Would recommend either the Workbench (better guns) or Infirmary (+max health) as your first upgrade.