If you’re going to be selling products to customers in a state with sales tax, you’re going to need a sales tax ID. Monthly or quarterly (depending on your state, and the volume of your transactions), you’ll need to report how much tax you collected, and pay that amount of tax into the state. Some point-of-sale systems will do this for you, so you don’t need to calculate everything on your own.
But how do you get a sales tax ID to begin with? If you’re in a state that collects sales tax, you begin by applying for a sales tax license. Nearly all sales tax states require that you be licensed to collect sales tax, though there are a handful that don’t. When you get your sales tax license, you’ll receive a sales tax ID. Alternatively, you may be told that your EIN qualifies as your tax ID, though you may still need a tax license.
Either way, you need to check with your state first, because this is decided on a state by state basis. And you should do it early in your company’s formation, just in case there are any delays.