Insecure guy, what do I do ?

what should a good looking 20 yr old guy do about being insecure about himself ? I have alot of girls that would stare at me or aproach ( sitting close etc ) but I never start a conversation and most of the time leave. I really get nervous around people, I have no idea why this is happening to me. I used to be normal during teen years. If I'm around people whom I know ( say in cl ) I'm being normal and have no problems whatsoever . I believe I am insecure I always think I might say something stupid. Some girls that I often talked to always tell me " so u are probably a little playa aren't u ? " and in my mind I laugh b/c it's opposite . Execuse my English I am Greek and thank u for any answers