I am so thin but my tummy I is fat?

Planks. Try planks. You hold yourself as if you were going to do push ups but don't go down. Hold as long as you can, then you can work on side planks to define the obliques.
Maybe medical issue. When a person gets fat, the fat is stored throughout his body, but more of it is the tummy area. However, if a person says he is thin everywhere else, but fat just in the tummy, there may be a medical problem. A big tummy in a thin person can be due to an abdominal mass, a pregnancy, malnutrition, very weak abdominal muscles, metabolic syndrome, etc... A doctor's evaluation can help figure it out.