I am happy to hear that you have already an idea of your niche. Great start!
In case you wish to use print on demand services this includes selling your own custom designs on products like T-shirts in your case. This way a supplier ships products directly to your customers on your behalf. In your case I would recommend trying out the Printify app or Printful. Here is a great article regarding the advantages and disadvantages of Print on demand services and AliExpress dropshipping: https://www.oberlo.com/blog/print-demand-vs-aliexpress-dropshipping
Most print on demand suppliers, such as Printful, allow you to design your own products which can be a fun experience. However, for running a business, the print on demand prices are relatively high. Eventually, you’ll need to invest in product photography to ensure you have great product photos. If you’re a graphic designer looking to monetize your talent, print on demand is likely your best bet.
What is more, it is worth mentioning that most print on demand suppliers don’t have the full spectrum when it comes to product selection. Therefore, it is advised to mix print on demand products with dropshipping of Oberlo Supply and AliExpress products. Having a mix of the two puts you at a competitive advantage. You can offer your customers a greater selection of products, competitive pricing and run a more profitable business.
Hope this clarifies things up!