What makes a person a stalker?

So, I'm re-writing my story... for like the billionth time. Anyway, I'm making Frankie a sort of stalker. Not a creepy stalker, but like a stalker who's fallen desperately in love with this girl and doesn't want her to spend time with any other boy.

So what all does a stalker do? Like during school mostly?

For example: Follow her around, stick notes in her locker and watch her to see if she reads the note, stare at her all class period, etc.