What is the excel formula to calculate a mortgage payment? also is
there a formula which can solve the amount of principal paid in a
defined period? for example, if I am borrowing $100,000 for 30 yrs. @
6.5% interest what are the monthly or yearly payments and also what is
the principal balance due after 5 yrs. (60 mos.) of payments?

There are fully worked-out templates already available for this.

Look at
http://office.microsoft.com/en-us/te...s/default.aspx
and search for "mortgage."