I'm surprised that no one else has asked this, but how long have the two of you been dating?
I see that you're at the end of your first semester of college, and it appears that he goes to college with you. I hope that you didn't just start dating this semester and you're already posting to forums asking about marriage. If that's the case (talking about marriage after 3 months of dating), then I'd say that you're definitely getting ahead of yourself.
Even if you've been dating before you went to college, I'd say 18 is too young. Yeah, there are testimonials about how it could work, but I think the statistical fact is that short dating/engagement periods correlate to higher rates of divorce. Also, younger marriage correlates to higher rates of divorce. Everybody thinks that they are the exception to the rule ("But we're really in love, I just know it.")
No matter what you do, though, get yourself a *useful* degree in college and make sure that you have some useful skills to get a job if need be. Even after the womens lib of the 60s, there are lots of women nowadays who get divorced in their 40s and have nothing to fall back on because their husbands brought in the money and they only had experience of being a housewife. And remember, nearly all of these women thought they were in love, as much as you do, when they married their husbands.
BTW, your boyfriend may make it to medical school, but it's a very tough and competitive road, and he could very well end up just getting a bachelors at the college that you both are currently at.