By ur name i get that you are male, and the question u asked i think u r asking bout a young girl u want to have sex with. Let me tell you first, that having sex with a minor (aged below 18 yrs) is a punishable offence/illegal.
what are you thinking fool?? consult a psychologist on this site.
Physically, you are capable.
But are you KIDDING? You're 12 years old, sweetheart! You do NOT need to have sex at this age. Please, just enjoy being a child before taking on adult responsibilities.