I'm not using USA settings but with v1.7.6.8, from Admin, down the left, you should see Improve / International. There are sub headings including Localization and Taxes. Check the taxes first to see if state rates are there. If not, go to Localization and import the localization pack for USA.
Once this is done, go back to Taxes and you should see all the state tax rates already available. Obviously check they're right.
These I presume then kick in when a customer from a given state places an order.
Hope that helps.