I need advice on how I can do this?

Honestly don’t know if this is the right place to say this but I’m gonna wing it.

So I’ve been with 2 guys before and they’ve always came but I never did. And honestly I’ve never had a sexual partner until recently. And my boyfriend keeps telling me to cum but he doesn’t get that I can’t.. like I’ve never came before and I don’t know how. Any advice on how I can teach myself to cum or if I’m doing something wrong? I masterbate but I’ve never gone as far as making myself cum. So any advice or help would really be appreciated