I lived in mexico for 43 years. I'm a us citizen. I just sold my first property, the house I lived in during the last 10 years. Do I have to pay taxes on it to the IRS? Any help would be much appreciated.

Yes.

"... Foreign source income. If you are a U.S. citizen with investment income from sources outside the United States (foreign income), you must report that income on your tax return unless it is exempt by U.S. law. This is true whether you reside inside or outside the United States and whether or not you receive a Form 1099 from the foreign payer...."

Publication 550 (2012), Investment Income and Expenses (http://www.irs.gov/publications/p550/ar01.html)