Do NOT tell her "how you feel". (That is un-manly wuss behavior!) Simply ask her out!! Do NOT use words like "I was wondering if..." or "maybe, if you're not busy..." Tell her what your plans are & that you want her to join you. Example: "Hey Mary, I got 2 tickets to the game on Friday, & I want you to come with me. It starts at 7, should I pick you up at 5:30, or 6?"
Do it like this! She doesn't get a yes-or-no question, her choices are 5:30 & 6. Learn this now while you are young!! You don't want to be an old geezer like your dad & asking for permission to go play golf.....
You can first of all start by psyching yourself up and getting close to her. After getting close you can then approach her and ask her to join you for lunch and to make it funny you can also tell her how shy you actually are.
It may sound a little childish but writing down your feelings is always the best way to express yourself if your too shy, write a note telling her how you feel and give it to her or mail it to her to avoid confrontation