Start here:
Equated Monthly Installment (EMI) Definition[
^] - it will explain what the Flat-rate EMI is and how you calculate it.
From that, it's pretty trivial to work out how to do the actual calculation: all you need is textboxes to allow the user to enter the loan amount, interest rate, and the duration.
Add a button to calculate the result, and handle it's Click event.
In the handler convert the user inputs to Decimal values (using
Decimal.TryParse[
^] and reporting problems to the user), then just use the calculation as shown in the first link.
Display the result in a Label or a read-only TextBox.