Total Mortgage (purchase price - down payment)
 You then must apply the mortgage rate to figure out the monthly payment. I would recommend checking out a website like http://www.interest.com to use their mortgage calculator. If you plan on using a standard mortgage, this should work fine (30-year fixed with 20% down). Otherwise, it becomes more complicated, and you should contact a mortgage professional.
Most people want to figure out their monthly payment, not just their mortgage payment, so you'll need to factor in some more numbers:
 Property Tax - most people pay this in their mortgage payment, but you can elect to pay them yourself twice a year, and not have them factored in to your monthly payment.
 Common Charges - condos charge monthly maintenance fees for the use and upkeep of common areas and ammenities.
 Homeowners Insurance - again, this number is usually paid in the monthly mortgage payment, but it must be factored in.
Overall, the monthly payment is commonly referred to as PITI (Principle, Interest, Tax, Insurance). With condos, the payment would be PITI plus common charges. I hope that this helps.