Anal for the first time

My boyfriend and I have been dating for 6 months and have a pretty good sex life. He always jokes about doing anal and says he’s only half kidding. It’s getting to the point where he wants to try it now. I’m really scared and I don’t want to do it at all. It’s out of my comfort zone. But I told him I would try it for him. I don’t know what to do about it and it’s stressing me out. Any advice?