Are you sure you want to calculate this by hand? If so, here's the formula that will figure out your monthly payment amount if you can provide the amount of your loan (P), the fixed interest rate (i), and the term of the loan in months (n). Just plug the numbers into the right-side of the equation, and you'll see how much your monthly mortgage payment (M) is over the life of the loan.
M = P(1+i)n i / [(1+i)n-1]
If you want to do it the easier way, use a mortgage calculator: