hey i suggest being REAL, check it out, people currently see you as being this silly never serious guy who could never seriously like a girl right? Well, if you want to stop that then simply show them a side of you that is the very opposite of that which people will most likely expect. Look at her straight in the eye, be real with the conversation and let her know how you feel. You dont have to be completely serious but you need to let this girl know you seriously have feelings for her. Remember, girls like guys that are real, know what they want and are not afraid to ask for it!
so just go for it and make sure she knows that you really care
and make sure you dont open with something like:
you "how much does a polar bear weigh"
her "umm i dunno"
you "its enough to break the ice!"
best of luck