Good that you're doing some research on the industry before launching your business.

If you're planning on using an existing platform like teespring, TeeChip or Viralstyle, there is no need to reinvent the wheel with your marketing. Review some of the most successful sellers on the platform and model their marketing efforts. Visit their websites, see their keywords and the niches they target. Also many of the platforms provide case studies. Another good source is YouTube and Udemy where entrepreneurs share practical info for cheap or free to get you going.

You can also find 3rd party marketing tools for fairly cheap that allow you to spy on successful campaigns. TeeSpy is tool (monthly subscription) that allows you to view winning t-shirt campaigns and the exact Facebook ads being used. The benefit is to save time, avoid frustration and make more money.

Good luck in your venture. If you decide to expand or want more specific guidance book a call and I'll share what I can for your success.

Cheers
Norm