You must have a reseller certificate from your state of residence. You can find out how to obtain one by conducting a Google search for your state.
When you have a completed state form, you then have to supply your Reseller Certificate to us. You can either email us the form with all pages, front and back. We will notify you when that form is applied to your account and you can then start purchasing tax-free. We do not have the ability to retroactively remove taxes on orders that have already shipped.