Where is your confidence?!
It's really a simple thing to walk up to someone (even if you are attracted to them) and start a conversation. If she/he doesn't know you, then it would be best to try to hold a few (in person) conversations first. If they do know you pretty well, then just ask them out. Go up to them & (take them aside if you want) & proceed to see if they would go on a date with you.
It is better if you have a plan. I would rather a guy ask me if I wanted to go to the movies as opposed to if I wanted to "hang out". It's always better to let a girl know what her options are. Maybe have a day in mind as well. I know if I was asked on a date I would want to know what day he was wanting to go on that date. Most girls want to know the day so they can make sure they don't already have plans. So have a day and event planned before asking. Be prepared!
Don't be like, "Oh, just whatever... whenever..."! That has desperate written all over it!