How can I not act like an idiot around my girlfriend?

ok so I've been going out with this girl for a few days and tonight was the first night I saw her. I realized im not in boyfriend mode. When im around her I get quiet and not talk to her and look dumb and talk to my friends, but theres also this one guy who talks crap on me all the time and we all hate him cept my one friend but anyway he always flirts with her and it was pissing me off, but the main thing is I want to know how I can not act like an idiot around this girl, I've known her for so long is there any secret to being more boyfriendish social able and such, I really like this girl schools starting next week and I need help : /

Be yourself, if this girl can't except you for who you are, she isn't really a girlfriend.

hey, there is no trick or secret to this situation you just need to be yourself and maybe talk to her a little more, if your not comfortable around her when your with your mates why dont you try to hang out with her by yourself and get to know her a little better in person. and start bringing her along with your mates. and this guy it sounds to me he has a thing for her, but I think she is only talking back to him is cause no you werent talking to her. im sure she likes you so start talking to her and being more friendly towrads her and im sure she will for get that guyb in no time she is probably really into her so go and talk to her hope this helps !!