ugh idk what to do

so i come from a very catholic, conservative family so my parents haven't taught me anything about sex really. they refuse to let me go on the pill (even though my period cramps make me miss school they're so bad). my boyfriend and i have talked about having sex in the future and i really feel physically and emotionally ready for it. the only thing holding me back is the fact that i am not on the pill and i am terrified that, even if we do use a condom, i'm going to get pregnant. i just recently turned 18 and have thought about buying birth control behind my parents' back (plus i am going to college next fall so i'll be away from my family), but i really just don't know where to start and if i'll be able to pay for it when i need to get more. can anyone point me in the right direction of what i should do??