Taxes for US citizens living abroad

I'm planning on using taxact to get my taxes done early at the beginning of next year. This year, I worked in the US and plan on making a little money (is USD at least) in brazil. How do I differenctiate my foreign earned income and my US income where federal and state taxes were already withheld. I don't pay income tax bc I don't make enough in the US and I prob won't make enough in Brazil to qualify for US income tax either. How do you all do it? Also, I read that the US and Brazil have a treaty where we only have to pay for one retirement and health plan at a time. Therefore, I dont have to pay social security and medicaid on top of Brazil's taxes. Or is this just included in the IRS forms?

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