Try reading this guide: https://www.askmehelpdesk.com/dating/guidlines-what-do-do-if-person-like-already-relationship-463250.html

How old are you?

If she has a boyfriend, that means that she's totally off limits and you need to not worry about making her fall for you.

If she were a single girl, then you'd just need to be nice, talk to her, and be yourself.

You should not tell her cause she would be stressed and you would be sad because she would reject you so stay apart and whe she is single you can try.. BUT TRY TO FORGET HER