First blow job.

Last night I tried giving my boyfriend my first blow job. He knows I've always been like nervous about it. He kept telling me there is no such thing as a crappy blow job but I can guarantee I ended up giving him one. It didn't even last long because of my gag reflex. I told him I had such a shit gag reflex. He said I ended up using my teeth too. I don't get how I used my teeth because I made sure I wasn't using them. What the hell can I do to give a good blow job and have it not interfere with my gag reflex? I feel like such a shit girlfriend for being shitty at blow jobs. Any tips or advice?