Sex with your bestfriend 😬

So I was talking with my best friend who is a guy and we started talking about sex and he asked me if I would have sex and do stuff with him. I think I want to but I don't want to ruin our friendship! We said that we'd be doing it as bestfriends and that we'll never be in a relationship because we are such good friends but I'm scared that it might ruin our friendship. I'm also not sure if I'm ready for sex because I've never done anything with a guy. Someone please help me and give me some advice. Thankyou xx