Full me in please.

So my kids father was never around. I'm so used to taking care of my kids on my own. Buying everything they need on my own. Well now I'm getting married (we don't live together yet), and I don't know how step parents are supposed to work. Please help me. It confuses me. I don't know why. He loves my kids and told me to get used to him buying things for them. But I never ask him to buy pull ups for my youngest cause I feel he isn't responsible to do so. So help please!!!! Thank you in advance.