A good page to start from:
http://www.irs.gov/Individuals/International-Taxpayers/U.S.-Citizens-and-Resident-Aliens-Abroad---Filing-Requirements
Requirements to file are found in Pub I1040: http://www.irs.gov/pub/irs-pdf/i1040.pdf
Income *gross income limits are:
Age <65 single $9,750...