I run a SaaS service for large POD merchants (basically, people doing what you're trying to do, many of whom are making a living at it).
Shopify does not give you organic traffic. You don't want to start with Shopify unless you already have a dedicated audience.
If you're just getting started, places like Redbubble and TeePublic are the easy way. Low margins, but it's "fire and forget". Amazon Merch used to be the hot ticket but it's pretty saturated. I would say that Etsy is the most popular place to make money in POD right now - it's a little more involved (you pay listing fees) but you control your pricing.
I've been in this space for about 6 years now. Happy to answer any questions.