The TurboTax software for tax year 2018 should be available in mid-November for the desktop editions and in December for the online editions. The TurboTax Freedom Edition is usually available by late January or early February.
However, due to the tax code changes there will be multiple updates to the software as the IRS issues new rules, forms, schedules and instructions. Completion and filing of a 2018 tax return should probably not be attempted until late February or early March of 2019.