How could 2 tests both be invalid? Are you sure you are using them right? Just to be sure, try to pee in a cup and dip the end in for 5-20 seconds (depending on what the instructions say). Negative does not mean invalid, it means not pregnant. I notice you are only 14, why do you think you are pregnant?
what do you mean by invalid? Do you mean that both tests had flaws and you couldn't get an accurate reading or did they just say negative? If you couldn't get an accurate reading from them then I would try another one in the morning as that is when your HCG levels are at their highest. If those test are invalid, then I would try a different brand of test.