I had my daughter right after I finished my junior year. It was hell. The pregnancy wasn't planned - my b/f (now hubby) and I had plans to finish high school, get married, go to college, have this perfect little life where we built our lives together, had the "normal" life, etc. But we got preggers, and everything kind of fell apart. We spent a lot of our 17-19 year old relationship fighting and bickering. We made it through it (he's finishing college this year, and I'm doing online-associate's), but we're the hard core exception. It was a FIGHT every step of the way. This year is our Jubilee (our 7th year together) and things are finally getting in order, lol.
So, do the smart thing, and avoid it at all costs. You won't lose much in your young relationship if you don't have sex. And if your hormones feel you 'must', get on good b/c - the ring, for instance, so you don't need to remember to take a pill each day.
I won't even go into the sleepless nights, the croup, the depression, the ugh...Just think about how much easier your life will be if you wait until you're in your early 20s and you've had time to get the craziness out of your system.