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 legally collect sales tax in your store.
To register, please follow the steps as described here.
You being a Printify merchant, we also recommend that you request a resale certificate from the state authorities. It is different from a sales tax permit as it allows you to buy products without paying sales tax on the Printify side, if you plan to resell those products and collect sales tax through your business. Sales tax only needs to be collected once if those are the same goods.
Once you've submitted a valid resale certificate to us, Printify won't collect sales tax from you.