Just to add to what Aria has posted:
Check out this older thread which links to articles and new research that shows that young women do have a pregnancy risk at any day of the cycle.
Knowing when you are fertile can hence be tricky business, especially for younger women, and just counting off the days of your cycle isn't enough. However, observing your cycles by the means of taking your basal body temperature every day and looking at the changes in your cervical mucous and cristallisation patterns in your saliva is -even if you might not want to rely on that method alone for contraception- a good way to get to know your body and your cycles.
If you have had regular cycles for several years, just like aria posted, and can be reliable in regards to charting and abstaining, Fertility Awareness *can* be a good method that you team up with condoms. It would mean that you use condoms (and also practise safer sex) each and every time you have partnered sex, and completely abstain from any sex with a pregnancy risk during your most fertile times.
For more on Fertility Awareness, check out Planned Parenthood: Ways to chart your fertility pattern. If you have been menstruating regularly for a couple of years and think you could handle the regularity of this method and are thinking about starting to practise it, I'd definitely recommend heading out to a bookstore to invest some money into a decent book on the issue.
"Through repetition the magic will be forced to rise."