A woman could find out she is pregnant approximately 5-7 days after her period is late, the earliest (under one condition: this number of days accounts only for women whose menstrual cycle is regular, e.g. gets her period every 28 days). The tests that might show the possibility of being pregnant are not 100% sensitive, so that's why you cannot know this important information earlier.

But, taking under consideration that 14 days pass from the ovulation (expelling of the egg, when she can get pregnant), until the day when she should get her next period (if not pregnant), plus these 5-7 days, all in all it takes approximately 3 weeks from the day of the conceptual/suspected sexual intercourse (plus/minus 2-3 days), to find out about a pregnancy via tests.

Moreover, pregnancy can be discovered via tests that measure the presense of a hormone called beta-HCG in urine or, more accurately, in the blood.

However, you and your partner might want to consult with an obstetrician/gynecologist, who can see the embrional sack ("see the pregnancy") with an ultrasound, a week after a late period.

As for the psychological part, keeping pregnancy a secret is always the worst thing to do, especially at a very young age. Talking with an adult (e.g. your family doctor, parents,...) might lead you both to the best possible solution in unplanned and unwanted pregnancies.

Finally, a combination of a condom and spermicides/BCPs are always a safer method to deal with contraception and getting STDs, although only celibacy is the safest one.