I like this girl in my school, but I feel too shy to approach her. My face turns red whenever she looks at me, and I just don't know what to say to her.
Be assured that your behaviour is absolutely normal, and that the~` are lots of teenagers of both sexes who face similar situations. Feeling comfortable when you are with a girl (especially one whom you like but don't know well) and being able to talk to her as a friend is a social skill that you will have to learn as part of growing up. Having good social skills would also mean that you are able to mix easily with others (whether they are boys or girls) and that you know how to modify your behaviour to suit different situations. Gaining these social skills to help you to mix with others is clearly an important developmental task, and is part of the social maturity mentioned earlier.
Social skills are developed only through practice. It may be easier for you if you first plan out what you will do when you next see that girl, and think of some things that you can talk to her about. Then when you do see her, pluck up your courage, force yourself to smile and say, "Hi!". At first, you will be shy, awkward and self-conscious, but this will improve as you gain more experience. The teen years is a very good time for such learning. The longer you delay, the more difficult it will be later on in your life.