How do I set up tax rates?

I am setting up an online store that sells printed materials for businesses. I currently serve customers in several states, and will soon be pushing into new territories. In my online store, however, I need to set up the tax rates for the Shopping Cart to calculate. How do I find that information? Should I charge my customers my local rate since this is where my business is, or do I charge them based on their zip code? I'm clueless!