You shouldn't view your outing as a 'date' unless you have made it clear that is actually is a 'date'. It seems like she views you as more of a friend than anything, and I am willing to bet that you are not the only guy that feels the same way. At 18 she either...
- doesn't realize that her flirting and close friendships are causing many guys to have real feelings for her
- is an attention whore
or
- flirts/befriends guys to use them to get material things
Either way your best bet is to be honest about your feelings and see what she says. However, be aware that there is a distinct possibility that she doesn't have the same interest in you. State your feelings, but don't expect anything. Then go from there.
...one can be sure of nothing until it has already happened...