A foul smelling whitish fluid is coming out from my belly button in small quantity. why? what should i do to stop it?

Many possibliities: There are many anatomic abnormalities that can cause a discharge from the umbilicus (belly button). Since your discharge is foul smelling and white, there is most likely an infection. Make an appointment with your doctor and, meanwhile, clean the area well with hydrogen peroxide on a cotton ball to remove discharge. If the area of you abdomen around the belly button gets red, seek care ASAP.