A medical fitness certificate is a certification that certifies that a person is fit or qualified to engage in a particular work activity. It is provided by government agencies or by government-recognized third parties.

So basically, to get it, you need to visit the medical college you are going to get in counseling have its own teaching hospital and you will have a medical check-up there itself at the time of admission. 

You can also visit the nearest primary health center or any government hospital. You can consult the medical officer there, and he's going to help you get the certificate. You can write a letter saying that you want it for MBBS admission.