well, i would recommend not getting married until after college. if that means not living together, so be it. people mature at different rates--and you may outgrow each other. also, college is a time of experimentation for a lot of people. it can be difficult to be tied to one person. in my law school almost 1/4 of the married couples were divorced by the end. the other ones were mostly older students who had been married several years before going to school. i think the numbers are worse for college-aged married couples.
i would not worry about this too much now. so much can happen between now and your high school graduation.
if you do decide to marry, don't forget the wedding expenses.
as to expenses and costs:
some people will borrow enough money to pay their tuition AND living expenses. i don't recommend this because you end up with a ton of debt upon graduation. it's better to have a part-time job for your living expenses.
costs of living vary depending on your location and your lifestyle. i lived on ramen noodles, drove an old car, didn't spend much on clothes/shoes/extras, and didn't eat out much or drink (alcohol is expensive) while in college and law school so my costs were kept down.
rent: anywhere from $500-$2000+/month depending on where you live. on campus housing can be cheaper.
car insurance depends on what kind of car you have. my husband and i have a ford explorer and a kia spectra: $120/month for car insurance. you can get quotes online.
health insurance: if you're single, you can typically stay on your parents' insurance while you're in college. if your parents don't have health insurance, your college will usually provide you with information on health insurance for students. if you work full time you might be able to get insurance through your employer. i think i paid $1300/semester for health insurance once i aged out of my parents' plan.
food is expensive.
don't forget utilities (water $30/month for us, electricity: $80/month), phone $120/month for two phones, travel/commuting costs $$$$$$$ with the current gas prices, entertainment, clothes, cosmetics/toiletries, cable/internet.
good luck to both of you.
word to the wise: whatever you do, DO NOT use credit cards unless you can pay off the entire balance each month. i cannot tell you how many people i know who thought charging a few dollars here and a fun trip there would be no big deal, only to find themselves with $10,000 in credit card debt with a high interest rate.