I'll make this short.

As soon as you get out of the house, turn your 'game' on. Start talking to the lady walking her dog. Say good morning to the bus driver. Smile and make eye contact with people on the street as you say 'Hello. How are you?' You must become the most social person in your whole city. Then, talking to girls will be a breeze and will feel all the more natural - the mental resistance vanishes. Do this everytime you step out of the house and you will rapidly improve as you learn to be you.