What do I put in the cost basis box

most likely the tax basis. if you don't know what that is how is anyone on a public forum!!!!! we would need more info to see if we can help. we have no access to your documents or tax return. 

Cost basis relates to the original cost that you paid for something.  In the case of securities there can be a number of variables such as stock splits, dividends, return of capital distributions, inheritance or company spinoffs.   

If this is a property, the starting point is the same.  You start with the original cost, but there are adjustments for improvements, depreciation, inheritance and other factors.

This article may give you some insights:  Cost Basis Basics.

@nick-346taxes 

Cost basis of what? Real property such as a rental? Securities? Bonds? Stocks? Something else?