If you co-owned it 50/50, then you each report half the sale proceeds, and you each report half the cost basis. You may qualify to exclude your capital gains if you owned the home and lived there for at least the last 2 years, but your father probably has to pay capital gains tax on his half.
While other ownership arrangements might have been possible, splitting the proceeds is proof by itself that you owned the house in whatever shares you split the proceeds.
Turbotax used to ask for the cost of the house when reporting the sale. This is incorrect, and I don't know if it has been changed. It should ask for the adjusted cost basis. The cost basis is generally what you paid for the house, but it can be adjusted by many factors such as the cost of permanent improvements, a partial inheritance (if your father co-owned the house with a spouse who passed away), a gift (if he gave you half the house instead of buying it together), and any depreciation that was taken or could have been taken for prior business use (as a rental, home office deduction, etc.)
The higher the adjusted cost basis you can prove, the less your capital gains. But if you are audited, the IRS will only give you the basis you can actually prove.
Start here to read about how to determine the adjusted cost basis of the home, https://www.irs.gov/taxtopics/tc703.html
