If you were legally divorced as of 12-31-17, even if you were married the other 360+ days of 2017, you file as Single or, if you qualify, Head of Household (HoH).
The main requirements for Head of Household are: a taxpayer that
1) is single or did not live with their spouse at any time during the last half of the tax year.
2) has at least one closely related dependent (usually a child) that lived with the taxpayer for more than half the year
3) pays over half of the support of the household expenses, including rent / mortgage, food, utilities...
