What should I say to this girl?

[Lizzie]

Don't not talk to her, but don't come off too chummy at first. DM her something casual, maybe a little joke about how you're a bit nervous and not too good at stuff like this, and mention that you were at the same camp and thought she seemed pretty cool so she doesn't wonder why someone randomly DMed her. Don't keep asking her questions either. Be interested, but also interesting. Respond to her answers and add your own (ex. if you ask her what kind of music she likes, respond to that and mention what you listen to. Ya know?)
Mostly, though, don't come off too arrogant, and be sweet and yourself. Good luck
~A girl her age