Okay how can I build up supply?

Isabelle
I try to pump in between feelings and I get a good 5 oz total but then when baby is ready to eat again I don't have enough for her 
Tonight I have to thaw a frozen bag ( I only had 2) because nothing was coming out of my boob 
She's 3 weeks old 
What can I do to get a supply going?
I really don't want to switch to formula
My baby is 11 lbs