are u late on ur period? I dnt want to preach to you on your age but sweety i had my first one when i was 15 and it was REALLY hard.Wait till your late and take one like they said in eairler posts first thing in the morning and if its positive good luck cause babies are ALOT of responsibility i am 31 and i still have days where i just say UGH...... and if its negative protect yourself!!!!!
Invalid would mean that nothing (no lines or anything) is showing up. This would mean there isn't enough or too much urine. Check the instructions. Also, if you are going to have sex at such a young age - please protect yourself with birth control, condoms, etc. A combination of several methods is better, but abstinence is best. You shouldn't have sex until you are old enough and stable enough to handle what sex can produce.
What type of test are you using. What does the instructions say, as far as it should say if invalid is a negative or not? Using your first urine in the morning is the most accurate.