A movie date is pretty casual (plus you'll be in the dark), so just whatever you'd wear on a regular basis. Don't worry about it. As cliche as "be yourself" is, if you go on the date trying to be all fancy and not looking like yourself, it's no good and may come off as trying too hard. Though on dates it's probably better to wear something not too "out there" and wearing something you know you look extra good in is always a good choice. :)
jeans, a tank and a cardigan, some flats. maybe put your hair back with a cute headband. keep the jewelry simple, just a simple pair of studs. keep your makeup simple as well, lip gloss nuetral eyeshadow and some blush. hope this helps and have fun on your date!