I forgot to file one of my taxes

[DoninGA]

Assuming that you are referring to not entering a W-2 on the 2018 tax return that has already been filed and accepted by either the IRS or your State, then you will have to amend the 2018 tax return to add the W-2.  However, do not start the amend process until you have either received the tax refund, the taxes owed have been paid and processed, or the IRS contacts you by mail concerning the missing W-2.

See this TurboTax support FAQ for amending a 2018 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/questions/4667233-how-do-i-amend-a-2018-return-in-turbotax