You should consult a CPA or tax specialist to fully understand your tax obligations.
Generally, you should charge sales tax when you have nexus. The main ways that you would have nexus are through physical presence or economic nexus. You can find a more complete list on TaxJar's Seller's Guide to ecommerce.
If you have sales tax nexus in a state, then you should look into getting a sales tax permit for that state.
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.
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.