The cost basis as indicated by @ColeenD3 would be zero if you have already fully depreciated the property under another business or rental use.
If the cost basis has not been fully depreciated, as you indicate in your recent question, but you have been using the property on your tax return as residential rental property since it was placed in service (not always the purchase date) then you should continue with the depreciation as usual. If it was always a rental then it was always 25.7y recovery period (number of years for depreciation period), using Residential Rental Property.
Keep in mind that land value must be entered so that no depreciation is being used for land because it is an appreciable (vs depreciable) asset.
If for some reason, you had this property listed as something other than residential rental property (nonresidential real property) in the past, then I suggest you indicate that property was disposed of with a date it was disposed of (the date you converted it to a residential rental building). No sale just disposition.
Then add the property back to your return under rental activity property. Adjust the cost basis for the depreciation already used for the different type and recovery period on prior returns. Keep track of your depreciation already used for future sale (keep a copy in your current tax files until you sell this property).
Please update with more details if you have more questions.
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