I am not a tax lawyer, but I have heard it this way. You need to pay capitol gains if you hold the coins for a year or more. If you spend them right after buying them it can be said you are using bitcoin as a payment network and not an investment. If spending/selling after a year try this formula.

First find the taxable amount:
Value of bitcoin at time of sale - amount paid for bitcoin = Taxable amount.
$660  - $12 = $648

Now depending on your tax bracket you may pay 0%-34%. Likely it will be 15% or 20%.
Taxable amount x percentage = Owed amount
$648 x .2=$129.6

So at 20% you owe $129.60 in this example.