Why am i so nervous to have a conversation with girls?

Lack of experience: Usually such nervousness stems from lack of experience. With more experience talking with girls you'll learn to relax and focus on the content of your conversation rather than on the thoughts that increase your anxiety (such as, "she probably thinks i'm stupid, and that's terrible."). Girls aren't scary, just as boys aren't scary. People are people. Don't scare yourself by unnecessary thoughts.