I'm a boy, so you may not get the BEST answer, but...
Try...practicing to a mirror or something. Don't kiss it. You'll be so accustomed to it, it'll be easier. Just pretend like she's someone you talk to everyday. Or even think of worse things that could happen. You could die. Just say it nice and firm. "I would like to go out with you." Or however you say itr...I've never been in that...situation. Make eye contact, but don't make puppy eyes, in those situations, it's just creepy. Don't be nervous. It's not the end of the world if she says no. You'll be all right.