Take her somewhere casual, but that you know has good food. Maybe a nice cafe, if you want to go super casual. Try to figure out what she expects. Is it a dinner or a lunch date? Is she the sort of person that likes to be paid for? If you don't know, try to offer to pay for the meal, but don't be pushy about it. Don't go overboard on spending, though, since it's just a first date; gifts and such are probably not a great idea yet. Tell her about you, ask her about her, if you're still getting to know each other. Beyond that, relax. It's a first date, not a marriage ceremony.