Don't think of it as "becoming shy," think of it as "becoming outgoing." You need a more positive perspective! You'll probably start out feeling a little shy, but the more you talk to girls, the more you'll build confidence & the easier it'll get. Start conversations with girls about something you have in common -- like a class at school, music you both like, TV shows or movies you both like, etc... Find out what the girl is interested in, then ask questions about it. Asking questions is an easier way to start a conversation than telling them things about yourself. Pay attention to their answers & they'll ask you questions, too. If you're funny, make girls laugh, that's always good. Women/girls love guys who can make them laugh! Good luck, be confident! :)
Everyone is different, but most people are able to overcome shyness when talking to girls by first being more outgoing in public. Once you find it easy to converse in a group, you'll have an easier time choosing an interesting topic to talk about with a girl. Just relax, be yourself and be friendly.
So what your asking is how to become confident. The trick is to pretend at first and then habitually you will become more confident, and it wouldn't feel like a difficult task anymore. Don't be tooo nice or try too hard to speak to them. Show them you are interested and be friendly.
Chicks dig confidence, ok so start off small, 1st day just say "Hi" and then keep on walking,. 2nd day say "How ya doin'?" and walk away. 3rd day "Hello, nice to see you again." Each day stay a little bit longer and she'll think you're confident. Before ya know it you'll be having a conversation.