IRS Free File
There are a couple of options on there. They have simple software you can use if your 2017 earnings were under $66,000. There are also free online fillable forms you can use if your income was over that, but they're a little more complex and you'll need to have your 2016 return available as a reference.
If your tax situation is complicated by some kinds of investments, or trust accounts and such, I think you have to go a more complicated route.
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~MissChess