Most Helpful Girl
First up, do not fret. It's like thinking up an excuse for the teacher to explain why you're late, they can tell when it's rehearsed and therefore untrue. (Not saying you're being untrue in asking her out)
Be yourself. Don't analyse every little word in your head before you say it, that is where a lot of people go wrong. Don't go for a cheesy pick up line, you could tell her how you feel about her, maybe make a cute little gesture you think she might like. Flowers or chocolate or something less corny, like if you've spoken before and she's mentioned a like at some point, take advantage of that information.
But be yourself. Don't try to make yourself look like more than you are, you want her to date you for you, not something you've made yourself out to be but aren't.
Good luck, and I hope all goes well ^-^0