When am I supposed to pump?

I’m having a hard time getting in the routine of pumping instead of directly putting the baby to the breast. I want to be able to have breast milk available in storage so I am able to go out or have my bf be able to feed him. He will no longer take any formula we give him since now he’s only getting breast milk. After he’s done eating my boobs are pretty much empty. And by the time my boobs feel ready to pump/ full of milk he wakes up to feed. Whenever I try to pump when he’s sleeping I usually don’t get as much as I know I can and by the time he wakes up I feel like I don’t have enough milk