early baby

I was due December 3rd but do to preeclampsia I had my baby at 36 weeks she was 6 lbs. 4 oz. an healthy . My question is does anyone know how to get beast milk to come in I barely get 4 ounces a day an would like to continue breast-feeding .my baby came home at 5 lbs. 11 oz. an is was difficulties gaining weight . Our next check up is in 2 days to see how she's doing . If she doesn't start gaining weight I will have to switch to formula . Also my milk is like clear almost it's definitely not whit like it was before is that good or bad?