Most of the fraternities and sororities take only one new member/pledge class per year, and that’s in the fall just before the first semester. A handful do small spring semester classes, but only a handful. So if you want the best Greek experience, plan to join in the fall.
Personally, I would take 15 hours, and a balance of hard and “cupcake” courses. Taking 12 hours is unnecessarily light and might delay graduation date on down the line. Taking 18 is very difficult with all the other changes a student experiences. Almost every student has the same basic course requirements and freshman year is a good time to take the basics and get acclimated to a strange new place, being on one’s own, etc.