How do i respond when guys talk to me?

whenever a guy talks to me in cl i'm just like "yeah" or "what" and try to make my eyes really big. i always feel really retarted when one talks to me, i mean sometimes i am just so surprised that one is talking to me that i'm just like "woah" but i am in 9th grade and still can't talk to guys!! i want them to think that i am funny and cool, but i am always just so "uhhhh what?" or just like idk , but i am such a nerd cause i can't talk to guys!! actually i am not a nerd, i hang out with the popular people at school