If you enter "0" as the cost basis, you will owe tax on the entire amount of proceeds (long-term capital gain).
If you can make a good-faith estimate on what was paid for the securities you sold, it will lower your tax on the proceeds.
You can research this online. One option is the Yahoo Finance website at: http://finance.yahoo.com/q/hp?s=YHOO
You will have to enter the stock symbol (“Get Historical Prices for: _________), and a date range.
This article provides additional information that might be helpful:
https://ttlc.intuit.com/replies/3300454
