I get REALLY nervous around my boyfriend?

My and my boyfriend just started going out. (i'm 13) I really like him and he really likes me. When I talk with him my voice gets all shaky so i try not to talk much. I also try not to make much eye contact. when we were just friends i could tell him ANYTHING and not get nervous. And another thing... he is my best friend's brother. Please, don't tell me i'm to young or to imagine him in his underpants. i'm not very imaginative. Please... help me get over this my problem...!!!