It sounds like you've already gotten some good advice, but I'll throw in my opinion/experience as well.
When I was 16-years-old (just about to turn 17) I started seeing a girl I didn't like at all. However, at the time I had self esteem issues and never so much as held hands with a girl. So despite my knowing I shouldn't I started dating her because she expressed a lot of interest in me because I was "really cute and funny" apparently. This was new to me and I was desperate for human connection as I'd never even been very good at making friends.
So about three months into the relationship we started having sex on a regular basis. The problem was that I didn't like it... at all, but again self esteem issues. I continued to date her until I was 19-years-old when we finally broke up. Over the course of our relationship I hated her more and more and then started hating myself. After we broke up I changed myself a lot by dressing better, keeping a better appearance, and being friendly and confident. Despite this I had problems when it came to sex and relationships. Just the thought of being with another person gave me weird little anxiety attacks.
Eventually (when I was twenty, nearly twenty) I started seeing another girl that I actually like... a lot. It took me a good while to start feeling sexually comfortable in that relationship because of all the negativity I associated with sex. However, once I got past that my girlfriend and I started sharing an awesome sex life and what not.
Basically don't have sex with someone you don't like, especially if it'll be your first time. It does more harm then good. Having sex with someone you actually really like is an experience worth waiting. I wish almost every day that I had waited.
Anyway, sorry to give you my life story and I realize it is a bit different than your situation, but I figured it may help.