For your current rental of the entire house, the date it was first placed in service is the date you made the entire house available for rent. Not the date someone signed a lease or moved it, but the date it was ready to rent and you began trying to find a tenant. Trying to find a tenant could be as simple as putting a "For Rent" sign if the front yard.
If you had been properly reporting the rental of a room in your house and taking depreciation expense, you would reduce your cost basis that you use to depreciate the whole house by the total amount of depreciation that you took on the room rental. In fact, that is how you should determine the cost basis for your current rental of the entire house.
When you sell the house, you will be subject to depreciation recapture tax on not just the depreciation you took, but the depreciation you were eligible to take. Unless you want to pay a 25% tax on a benefit you never received, you will need to correct the prior depreciation.
You need to file Form 3115, Application for Change in Accounting Method to retroactively claim all the depreciation that you should have taken on the room rental. You can prepare and file it using TurboTax; however, it is not a simple form, so you might want to consult with a local tax professional.
Finally, I strongly recommend that you use TurboTax Desktop since you have rental property.
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