Some of the information provided by @DianeW777 is not complete and could easily be misunderstood.
For rental property reported on SCH E, start up expenses are not allowed. Start up expenses are allowed only if the rental qualifies as a SCH C business and is actually reported on SCH C. It is not common for long term residential rental real estate to qualify as a SCH C business, as it can be very difficult to meet the requirements to qualify as a SCH C business. For a "down to earth" explanation, see https://www.landlordstudio.com/blog/schedule-c-vs-schedule-e
Cost Basis - Generally, this is what you paid for the property when you originally purchased it. You add to that cost basis anything you paid for property improvements after your original purchase. Understand that there is a difference between property improvements, repairs, and maintenance expenses.
Depreciation - Land is not depreciated. Your cost basis for depreciation is the value of the structure, plus the cost of any property improvements to that structure that you paid for after your initial purchase of the land. The program (not you) will help you to determine the value of your land and the value of your structure including property improvements.
Rental property uses the mid-month convention for deprecation. So depreciation starts on the 15th day of whatever month you place the property in service. Basically, it's "in service" on the first day a renter "could" move in. That's usually the day you put the FOR RENT sign in the front yard. So it doesn't matter if your in service date is March 1st or March 31st, depreciation starts on March 15th. But the actual date does matter for other rental expenses.
Generally, any costs incurred in the process of preparing the property for rent for the very first time are not deductible. This does not include property improvements. Property improvement costs are added to the cost basis of the property, and it does not matter when that property improvement was done, so long as it was "after" your initial purchase of the property.
