How Do I ask Her Out?

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I'm 16 and I absolutely adore this girl. She's very pretty,obviously alot of guys talk to her, but it's hard for me to understand, none of them have actually asked her out. Now, she did like this one guy but he was rude, arrogant and left her alone at a dance. Anyhow, we've grown close over the year, she tickles me, hugs me everytime she see's me. We'll we've talked since schools been out, yesterday, she Dm'd me and we chatted a bit and she said "take me to AC " well obviously I can't,for a number of reasons, and I'd like to spend time with her for July 4th but I won't be here, so My question is, I want to ask to go to the Zoo when I get back, how do I go about this, I'm a little nervous, I know she'll say yes, but I have to ask over twitter, because my phone is broke.
I'm really a shy guy, so what should I do?