1) add up your income and investments
2) determine how much liquid (i.e. readily available cash you can have for a down payment)
3) add up all your bills and debt
4) determine your monthly left over cash after all expenses and bills
5) this is what you have to put towards a mortgage (don't forget if you pay rent now to NOT include that as an expense b/c that money will now go to your mortgage)
6) look at that number - and cry
but seriously, look at the interest rate and figure out what that equates to - don't forget to consider what you are placing down as a down payment and what your credit score is as that will affect your interest rate - and please PLEASE do a 30 yr fixed. trust me.