Don't go this route. Start with a decent used geared bike. The rear dropout spacing won't work for geared bike wheels. There are no stops for cable and housing, there is no derailleur hanger. You'll need a new rear wheel, derailleur, and shifting system which will probably cost more than that bike... or a decent used geared bike.

Starting with a fg/ss bike when your goal is a geared bike is a Bad Idea.