How much am I suppose to weigh?

If I am 161 cm tall and I am 12 years old how much am I suppose to weigh?

Better to ask your doctor on you next visit. S/he can look at your body type and bone structure and make a more useful recommendation than any chart you'll find online.

In the meantime, eat healthfully and exercise regularly.

Medical professionals use the Body Mass Index (BMI) to determine whether your weight is within healthy boundaries. BMI Calculator:

http://www.docshop.com/education/bariatrics/calculator