I'd ask her at the theatre- maybe something like:
"You get the tix, I'll get the popcorn and drinks,"
or,
if you guys are going out for drinks or whatever after/before, see if she'll split the cost. Ie- she pays for the dinner/drinks, you buy the movie.
But if it's just a movie, and it's a first date, I'd ask her.
If you're cool with it, and she doesn't seem like she's just using you for the movie, I'd pay for it. Then, next time, she can buy:
"Hey, I'll get this one, you can pay next time, cool?"