Orgasm Questions

So I'm 17 and I just began a relationship with a 20 (soon to be 21) year old ( that I met 3 weeks ago) We had sex a few days after we met each other and so we had a discussion about things we want to do and try. I mentioned that I had never had a orgasm. The last time we did it we had 2 rounds, 1 was for him and the other for me. I didn't orgasm but I think I was close to it but I knew he was wearing out. Basically, my question is how do you know when you're about to and when you have an orgasm? Also any tips to give him? I don't explore my own body because it really doesn't do anything for me but I don't just want to leave it up to him.