First off, a year lower is no prob, girls like guys who are older than them.
Now, here's what you should do.
Ask her out to another movie. She likes you. Or at least, that's what her body language in that movie said. Ahe wants you, If you dont ask her out soon though, she'll give up. So just ask her.
She won't say no. She's given you indicators that she likes you, conscious ones, so she'll understand that you have reason to think she likes you. Now, when you ask her out, if she says she'll need to check with her parents to make sure she isnt busy, ask to trade phone numbers.
During the movie, put your arm around her shoulder. If she leans in toward you, that's a really good sign. Now stroke her hair. If she doesnt object, she's very comfortable with you.
Here's how to go for the kiss, if she shows she's very comfortable. After the movie, outside the theater, give her a hug. As you begin to let go, stop with your hands on her hips and look into her eyes and angle your head towards hers a bit. If she makes eye contact and holds it for half a second and doesnt move away, kiss her, she wants you to. Just do it, or she'll think "This guy could've kissed me and didnt have the nerve".
Now, let's say she doesnt want to go out with you, either you waited to long, or messed something up, know this:
Something about you attracted this girl, in that movie theater, she felt a connection. You aren't some slob who will never go out with a girl in his life, you're someone who can attract a girl. View whatever happens as a learning experience. If you succeed congrats. If not, find out what you did wrong, you can learn not to do it next time.
So that's my advice. Good luck my friend! Any questions, PM me.