Do NOT do it over a social networking site, or text message. Make your best effort to ask her in person. The manner in which you ask someone out really depends on the level of your relationship, so I’d start it off simply and approach her when she’s alone (it’ll be easier if you are shy, anyway). Don’t make a date-itinerary. Let things be flexible, because the unpredictable dates are often the best ones. 
Good luck!