If I am a US citizen and was a Au Pair in a foreign country do I need to claim any income on my taxes.

It depends.  As a US citizen you are taxed on all income worldwide.  You income earned in another country is generally treated the same as that earned in the US.  Having said that there is a foreign income exclusion available if you meet certain criteria.  Generally if you are:

• A U.S. citizen who is a bona fide resident of a foreign country or countries for an uninterrupted period that includes an entire tax year,
• A U.S. resident alien who is a citizen or national of a country with which the United States has an income tax treaty in effect and who is a bona fide resident of a foreign country or countries for an uninterrupted period that includes an entire tax year, or
• A U.S. citizen or a U.S. resident alien who is physically present in a foreign country or countries for at least 330 full days during any period of 12 consecutive months.

Then you may be able to exclude some or all of your foreign earned income from US taxable income.  There are other rules that may apply as well.  Here is a link with an interactive interview to help you determine the status.

TurboTax will help you with the exclusion amount.  In the "Less Common Income" topic "Start" the  "Foreign Income and Exclusion" topic.