Most pregnancy tests will not yield an accurate answer until 5 days before a missed period. That means, that if you have sex on week 2 of your cycle and you expect your period in 1 week, you can take the test after a few days.
Since you are obviously young and having trouble, it is important to learn contraceptive (anti-pregnancy) methods that work for you. You are allowed to go to planned parenthood and get the birth control pill without them telling your parents. You must still use condoms to protect against STDs, but the pill (taken EVERY SINGLE DAY) is the best way to protect against pregnancy.