Gyno,
Goes on line 13 of your tax statement.
You will need to fill out a Schedule D and that should provide you with your net gain.
It is divided into short-term and long-term gains and losses.
It should be fairly easy to find your cost basis from your account.
I use Sharebuilder as well and it may take a little time, but is pretty simple math.
Let me know if I can help.