Generally, you should charge sales tax when you have a nexus. The main ways that you would have a nexus are through physical presence or an economic nexus. You can find a more complete list on TaxJar's Seller's Guide to ecommerce.
If you have a sales tax nexus in a state, then you should look into getting a sales tax permit for that state.
Physical Presence
Sellers always have a physical presence in their home state, meaning you probably need to register for a sales tax permit in your own state.
You may have a physical presence in another state if you have an office/second location, an employee working in a second state, or if you have inventory stored in another state, such as with Printful's warehousing services.
Economic Nexus
If a seller passes a state’s economic threshold for total revenue or number of transactions in that state, they are legally obligated to collect and remit sales tax to that state. A full list of states and their thresholds can be found on TaxJar here.
