I plagiarized, and i got caught!! help!?

i'm in middle school, and today my teacher told me that i plagiarized two sentences on my research paper, he told me that i'm going to get an F on my report card and a "U" (unsatisfactory) on my work habits and cooperation, i'm not sure if i'm getting it on both. he told me that he's calling my parents to have a conference tomorrow, i'm really worried, any advice?, i never been in this situation, what should i tell him?-tell me ASAP-thanks!