Step 1: Learn at least a little something about your girl
How would you describe her? Edgy? Nature-y? Soft and romantic? Career/goal-oriented? Delicate or bold? Shy or outgoing? Funny, quirky, serious? Even just one or two descriptors will help.
Is she allergic to flowers? Worth finding out. If you don't want to be obvious and ask, find some flowers when you're out together for her to smell. That'll give you a good idea.