Just be honest and say, "hi." Since this is the first time you've met, treat it as such . . .
"Hi, I've noticed you and wanted to say hello for weeks, so hi. I'm Molly and seeing you daily I just knew I had to meet you, etc., etc., etc. ------"
The key thing is be open and honest even if you are nervous. So what? You are still you and will be after your introduction. Regardless of the outcome you will have done a very human and humble thing, i.e. honestly made an outreach to another person. How she reacts is how she reacts and all about her.
One final thing, have fun!!!!
Rachel