Once you’ve determined that you have nexus with a given state, you should register for a sales tax permit as it is required in order to collect sales tax.
To register, please follow the steps as described here.
Being a Printify seller, we recommend that you request a resale certificate from state authorities. It is different from a sales tax permit as it allows you to buy products without paying sales tax on our end if you plan to resell them and collect the sales tax through your business. This means that Printify will not collect sales tax from you.