Well, first of all, if she is still in high school, be careful if she is under 18. Since you are 18, you don't want to get into any issues with her being underage.
Second, it is really hard to make it a "first date" when all you are doing is hanging out at your house and watching a movie. Aside from the potential hookup, of course. I would just keep it low-key. Make it as casual as possible, but be sure to ask her out on a real date as well, so she doesn't think you are just after a fast piece of ass.
Good luck.
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