How to add fingers?

Okay so I'm 16 and I like to masturbate but there's a problem I've encountered. I've been talking to my boyfriend and he wants to try fingering me but I'm SUPER tight. I've tried myself and I can only fit one finger in there, that's it. Luckily my boyfriend is okay with it, it's actually a turn on for him to know that I'm tight as hell. But then it scares me to think what am I going to do when I'm ready to have sex?! When I try to put 2 it obviously hurts so what can I do kind of stretch it out?