quick way to calculate interest on mortgage

[William MacBride]

Whats the quick formula tom calculate monthly interest payments for a fixed rate mortgage? i.e. just to start with any total balance and quickly figure out what it would cost per month and/or per year with interest and pricipal.
Also I wanted to ask, with fixed rate mortgages, the payments stay the same, so does that mean that more and more of the principal is being paid by the payment and less and less interest time goes on? I.E. the interest gets charged on the remaining balance, not the total balance, but the payments stay the same so more goes to pay off principal? I'm having trouble figuring out how to calculate this.
I realize this is really basic but I'm new to this.

Thanks a lot,
Will