Figuring out percentages is pretty simple once you know what percent means. Percent means a portion of something divided into 100 pieces, or X/100.
In your case you want 8%, so we do the math 8/100, and we get 0.08 as the answer.
Now that we have actual numbers with which to work, 8 % of $700 is calculated as 0.08 * 700, which is 56.
So the sales tax on the item is $56, and when we add that to the item's cost ($700 + $56) we get $156.00 as the total.
Reuters is reporting that Nassau County in New York is expected to lose some $30,000,000 in sales tax due to the effects of "superstorm Sandy."