Relax and keep it simple. Just go up to them and introduce yourself by saying "Hi" or whatever (make sure you have something to say after hi!). No need for crazy lines, keep it natural. You can make some small talk and if you want to flirt with her you can compliment her on something you genuinely feel should be complimented.
If you get nervous cuz the girl is really hot, you can do what I used to do back in high school - go google the hottest actress you can think of and keep staring at her picture so that when you talk to the actual girl, she seems less hot in comparison (hey don't laugh at me. i see you laughing. i see you!)
Good luck dude. Go get em boy
Yesterday you said tomorrow... ^