If you know the amount of your mortgage and the interest rate you will be charged, a mortgage calculator will calculate your monthly payment of principal and interest.
A word of caution: Mortgage calculators calculate principal and interest ONLY. You will have additional monthly expenses such as escrowed property taxes, home owner's insurance, and PMI (private mortgage insurance--if your down payment is less than 20% of the purchase price).
Click on the link below for a simple mortgage calculator. The home page of this website offers other useful calculators, too.