Feb 20, 20151. Have a legitimate reason why you need to contact her. Make plans. Tell her you want to get drinks sometime. Invite her to a party. Exchanging information now makes sense. Don't just randomly ask for her number. That's boring and creepy.
2. Ask for the best way to get a hold of her. Literally say "What's the best way to get a hold of you?". Maybe it's a number, maybe it's Facebook, maybe it's Snapchat. Let her decide. She'll almost always say "Oh, here's my number".