That's a huge question. Why do you want this done? A Doctor/Surgeon would never do this without sound medical reasons such as it being a 'life and death' situation. If the reason you are asking this is because of painful periods you need to investigate this much further and look to other possibilities first. Severe period pains and cramps can be scary enough to make a young person want to take extreme measures, yet if they did maturity would no doubt give them cause for regrets, e.g. loss of ability to conceive.
Be patient (assuming you ARE the 12 year old girl herself), and look into the other options such as 'Mefenamic Acid' and 'Endometriosis' before taking radical action.