Missed cycle

Okay so i don't exactly know when my cycle is due to start because i haven't had a phone and i don't know how but i lost track of my period. I usually start the day my sister end and she has been off for 3 to 4 days. I had sex with a dude and tomorrow will make a week ago. He is freaking out that im pregnant and i wasn't so worried until i though i missed my cycle. Im very underaged to be exact im only 15 and i don't know how to get a pregnancy test or how to tell my parents if i am. I need advise from some. (Please don't judge i know i messed up)🤦🏿