A barn is an asset. You will enter your expenses to build the barn for the basis. A farm building is usually depreciated over 20 years. Follow these steps:
Answer yes to assets
Answer no to $2500 or less, I am sure the barn cost much more
Continue to Describe this Asset
If it is a single purpose structure, you select the first option. Barns are hard to qualify as single purpose so that may not be correct for you.
You may need to choose the last option and go through those questions. You will select 20 year depreciation.