While the facts are limited, if all you did was invest some $$ in Coinbase and then sold them, the answer is fairly straight forward. This would be handled just as if you purchased some stock and then sold it.
You know how much you invested (basis) and you know what you got back (proceeds). This is just reported on form 8949 and Sch D. The only other item you need to input is the date(s) acquired and then sold.
If you had multiple transactions, then this would not be quite as straight forward, but not all that difficult. However, as noted, you would need to obtain some information from Coinbase to understand each transaction and then input as noted above.
The IRS will be looking for this as there is a specific question regarding whether you have any crypto currency transactions.