@bigbryce31186
The 1st step is to ask your T shirt producer what type of files they prefer.
There are two types of digital art
Vector and Raster
Photoshop and other image manipulation software creates raster art, AKA pixels.
Pixel art can only be scaled up to its original created resolution but can be scaled to smaller sized art without any degradation. Most T shirt companies will take raster art like .jpg, .tif, .PNG and retrace them to create vector art. This sometimes costs extra money for "pre press" work.
Gimp is a free image tool that is as good as Photoshop. It's as if you took PS and all of it's functions and shook them up. Learning Gimp is frustrating if you already know Photoshop.
As for Vector art, Illustrator is the industry standard. It saves vector art as EPS, Ai, SVG files
Vector art is points. you can scale them infinitely without any art degradation. InkScape is a down and dirty version of illustrator and it's free. May be mac only.
If you are starting from scratch learning software, again, talk to the production people. Call CustomInk.com and see what they say. Raster art image quality is often measured in PDI or PPI. Dots/Pixels per inch. You can ask what DPI files they need at 100% size or ask if they prefer vector files