Thanks for your question. I have a Print-on-demand guide which includes from the very first step to set up, and I think it would be very useful for you if you want to start to build a POD business.
Check the POD guide here.
And if you have time, you can check some templates which are suitable for the POD store:
In these templates, you can add many products.
I hope my answer can help you in some ways. Let me know if you have any questions. Cheers!