Why am I so weird with boys?

SO, I am a 15 year old girl, freshmen. All my friend know how to talk to guys like they talk to me, they go about it with such ease. I can talk to guys, kinda. but not like I am normally, I get all seized up and afraid to say the wrong thing, look the wrong way. I want a boyfriend really bad but I'm not friends with any guys and I don't talk to any guys often. My friends do and it makes me feel really sad and left out. I hate that I can't talk to guys. How do I act myself around guys, start up conversations, be myself, etc. etc? help please:(