First of all, calm down! I know how a pregnancy scare feels, and it's really terrifying, so just take a deep breath and drink some water. The world is going to keep on turning, including your own.
Now listen to me. How long ago did you have your last period? Two weeks? Yesterday? Depending on when you get your period, the chances are higher or lower that you could have become pregnant. When you ovulate, about halfway through your menstrual cycle depending on how long it is, that's when the ovary releases a fresh new egg into the Fallopian tube. This egg is ready to be impregnated by a sperm cell. 12-24 hours later, it starts to die. Near the end of your cycle (the week before you get your period), or near the beginning (right after your period ends) there's really not much to fertilize. Every day after your ovulation day, it becomes less likely that you conceive. If you had sex at those times you're probably pretty safe. Not that I think you should be doing this again, but just, you know, in hindsight. The luteal phase of the menstrual cycle (the time in between your ovulation day and your next period) is about 14 days for most women. If your last period was about two weeks ago, that is if you have an average 28-day cycle, then that was your "fertile window" and you have a higher chance of being pregnant.
Now, when I was your age, I didn't track my periods, and I thought it was dumb to do so. I only started that when I was trying to get pregnant. But whether you're "TTC" or not, it's useful because then you know when they're coming. It's hard to do on a calendar, so get an app (I recommend "Fertility Friend") and it tells you when you ovulate, the days you're most likely to conceive, how long your luteal and follicular phases are, and when you're going to get your period. There are patterns in your body you probably don't even know about.
As for Plan B, that only works for about seventy-two hours. So if it's been longer than that, plan B isn't going to do anything for you. If it's been less than that, see if you BF and you can pool your money to buy it. Ask a friend to lend you money if you need it. Heck, you can ask your mom for the money and just tell her it's for something else (tell her you want to buy a new shirt or something).
If you DO know when your last period was, use that as a judge; two weeks ago, perhaps reason for worry, very recently or you are expecting it soon, don't worry so much. Also! Wait for your period to come. I call periods "nature's pregnancy test." If it doesn't arrive, go to the pharmacy and buy a pregnancy test. They don't require you to be eighteen or older. If you're embarrassed just say it's for your sister and you're hoping you'll be an aunt or something like that. Or use the self-checkout. And while you're at the pharmacy, buy yourself some condoms, too. Or have your BF buy them. Start tracking your periods if you don't already.
Speaking of your boyfriend, assuming you haven't already, DO share your concerns with him. Try to break it to him gently, of course, but this is less scary when you have someone to worry with. Talk with him about what you'll do if that pregnancy test is positive. It takes two to tango.
P.S. Other Yahoo Answers people, I'm pretty sure she is not looking for a lecture. She's asking if she's pregnant and if so what can she do. Stop judging. Like YOU'VE never done anything irresponsible or regretful in your lives. Let he who is without sin cast the first stone. Get off your goddamn high horses and be helpful for once.