social networking website requires

i want to build a social networking site for youngsters of a specific city ...it wil not be just a social networking .. but evrything that attracts youngsters ..like best hang put places in the city ... hotels,menus... buy n sell anything in the city .. in short i need a combination of social networking plus buy n sell plus information about da city ...n just everything about the city .... i am a beginner so i cannot afford much money ... bid as less as u can .