Want to move in with my GF in 9 months, need to build credit, how?

Never had a credit card, my credit score is low 600's, looking for places in NYC area, what's the best way to start to build credit so I can rent a place under my name? Also anyone know best options so i have a likelyhood of getting accepted. Currently live in a nice place in the city and while i'm the lease, the bills are paid primarly by my roommate.