By default, an LLC with one owner is treated as a disregarded entity. You file the business income and expenses on Schedule C of a personal tax return. The net profit flows to schedule SE to calculate the self-employment tax, and the net profit also flows to form 1040 where it is combined with any other income, deductions and credits. The LLC owner can file a form to make an election to be taxed as an S-corporation, which would be on form 1065. At this point, the election could not be retroactive to January 1, 2018 or January 1, 2019. It could be retroactive to January 15, which would be a bit of a mess for your 2019 filing, or it could take effect January 1, 2020, which seems most sensible. I can't advise on the tax consequences of being taxed as an S-corp versus a disregarded entity, you would need to speak to an accountant.
As a disregarded entity for 2018, filing a schedule C, and because you have startup costs but no income in 2018, you don't file a schedule C for 2018. Your startup costs are reported in the first year you report business income. Depending on the kind and amount, they may be expensed in the first year, or amortized over 15 years, or some of both. s
And of course, if you are not a US resident, you would file a form 1040-NR which can't be prepared in Turbotax, you would need another provider. If you are a US resident for 2019 (lived in the US at least 183 days or otherwise meet the substantial presence test) then you file a normal US tax return.
