Hi Presley, if you're looking to create designs for t-shirts, I will suggest Inkscape.org. It's is a free, and open source, vector graphics editor. Also, Paint.net was great to use at one time for me. I liked Paint.net because I was able to add a PSD plugin that allowed .psd (Photoshop) files to be loaded. It's a nice feature when you have to edit .psd files all of the time, but don't want to purchase Photoshop.
You should also have a look at TeeSpring. The T-shirts you design may become a viral craze that pays. TeeSpring is super cool! You can create your shirts there, to be sold in an online marketplace. I hope that helps you out too!?