It varies from state to state and within those states, from country to county and even city to city.
For example, in Washington, the state sales tax is 8.9%...BUT it is higher in some cities and/or counties if the citizens there have voted for it. Like, in Vancouver/Clark County WA., the tax is a bit higher as they use it to pay for extras, like roads and school programs.
Call your chamber of commerce, they should be able to help you.
To Scott (above)...there may not be taxes on iTunes, but there are additonal fees. Anyone who has used an iTunes card can tell you that.