For this question, I am assuming that the LLC you need to file for is going to be included on your personal return. If the LLC is a completely separate legal entity, they will need to file a separate federal tax form and you would then amend your return to include the flow through income from the LLC on your personal return.
There is no fee for amending your returns if you prepared them originally with TurboTax.
If you did not file your original return with TurboTax, you would need to use our downloaded software or online software to recreate the original return that was filed, and then select the option to amend the return. These would have to be filed by paper, since the original federal return was not filed through Turbotax.
Here is a link to our desktop software, and the cost will depend on what other information is reported in your return.