You want to go to places that wouldn't be too noticeable. Like if there is a highs chool by either of your houses.. go there and just chill out where nobody can see you. Or in the woods, on playgrounds at night, in the car, if you go in the mall you might be able to sneak in a dressing room together and do it there, or just tell your parents to lighten up because if you are 18.. they have no say in what you do and tell them that if you are not even having sex they shouldn't care.. at all.
Good Luck, believe it or not.. your parents are trying to help you out. It seems like they are just being rude and that kind of thing but seriously they don't want you to get pregnant or anything.