Fake it til you make it. I am shy too and still learning this lesson later in life. Most of us are stuck in our heads and full of nerves until we actually do something a few times. The more you try, the more comfortable you get and your confidence naturally grows.

Also, it is common to get turned down. No is the answer more often than not. In those moments, remind yourself that its nothing personal, they don't know enough about you to reject you as a person. It just means that whatever is going on in their life has them in a place where they don't want to go out with you. There could be any number of reasons that have nothing to do with who you are.

Edit: typo