I may be biased as I am in the print-on-demand business, but from personal experience I encourage people to really decide if the actual manufacturing of products is something you're interested in.
It sounds great to just go buy a heat press and start printing t-shirts, but it's really not as easy as it sounds. Before you know it, you'll be spending all your time figuring out the heat press, transfers, inks, etc. and left with little time to market your business, drum up customers, and create designs. Sometimes it makes more sense to leave the manufacturing to someone else, while you concentrate on all the other aspects of the business (marketing, hiring employees, creating designs, evaluating blank garments, taxes, accounting, sales tax, e-commerce, credit cards, etc.)
Before you jump in, make sure you have given it a lot of thought! Running a business is not as easy as it may appear, and you may be more successful if you concentrate your efforts to one area!
Just some food for thought.