I am a Muslim sister who happens to like a Muslim brother, but he doesn't know that I like him. What should I do?

All perfect praise be to Allaah, The Lord of the Worlds.  I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

First of all, we warn you against using such love as a pretext to committing Haraam (forbidden) actions such as looking at alien (i.e., marriageable) people or being in seclusion with them. In addition, if you are not married, it is lawful for you to express to this man your desire to get married to him provided this is done in a way that is safe from temptation, trial and misconceptions. If he marries you, then that is what is sought, as marriage is the appropriate cure to love. It is reported that the Prophet , said: “Nothing can be more beneficial for lovers than marriage.” [Ahmad and Ibn Maajah]

However, if marriage cannot come easily with this man, then you should forget about him and stop your feelings from going beyond the Sharee'ah defined limits, as that could induce sins.

Allaah Knows best.