The standard formula
M = P x [ r(1 + r)^n ] / [ (1 + r)^n - 1 ], where P is the principal, r is the monthly interest rate (annual rate divided by 12) and n is the number of monthly payments.
For a $250,000 loan at 6.5% over 30 years: r = 0.065/12 = 0.005417, n = 360. The formula gives roughly $1,580 per month.
Why early payments are mostly interest
Interest is charged on the outstanding balance. Early on, the balance is at its highest, so most of your payment covers interest and only a small slice reduces the principal.
By the final years the balance is small, so almost the entire payment goes to principal. Over 30 years at 6.5% you pay about $319,000 in interest on that $250,000 loan - more than the loan itself.
Three levers that actually help
Shorten the term. Moving from 30 to 15 years typically saves six figures in interest, at the cost of a higher monthly payment.
Make extra principal payments. Even $100 extra a month on a 30-year loan can cut years off the term because it attacks the balance directly.
Refinance when rates drop meaningfully. Compare the closing costs against the monthly saving and work out the break-even point before committing.