You can tell a person how you feel, but it is best to show it. You should do so slowly by gauging the other person and how they react to you. First of all, girls don't know if a guy likes them or not. You have to show that you are interested in her. There is a saying that people like those who like them. If you are open to like her, and she becomes aware of it, then all of a sudden you become attractive to her. The best advice I can give, make her feel comfortable. If her feelings are not reciprocated, then leave it alone, if you proceed any further then you become a nuisance. If she does reciprocate, take your time and allow something to develop. Don't rush it.
Keep this in mind. There is a saying: "you don't own her, it was just your turn." If she wants to leave, let her go.