I realize that a lot of you T-shirt designers are new and there are numerous design program options out there.
Photoshop and Illustrator are the industry standard. I will advise anyone who wants to design to go with the best. These two are the best in my own opinion and they are the only people that I endorse and they are the ones I usually use when I create my videos.
Whichever one you use will depend on your design style and what you want to do.
You can learn Photoshop in detail from my tutorial where I teach Photoshop (the link is below).
Photoshop is based on pixels. Pixels are little squares of colors that come together to make a whole photograph is. It is a digital representation of real-life things made by these little dots called pixels.
DPI means dots per inch. This means the amount of dots in every square inch. The smaller the dots, the better the picture quality.
Illustrator uses vectors where Photoshop uses pixels. Illustrator has dots and anchor points that give you the ability to manipulate shapes and colors. Illustrator works like you are drawing and with certain tools you can actually draw. You can check out my Illustrator course too.
The basic difference between these two programs is that Illustrator uses mathematical equations known as vectors.
Designers who do a lot of drawings and text designs should use Illustrator. This is because you can take any basic text and turn it into anything using Illustrator.
One thing about Illustrator is that, no matter how much you zoom in on the shapes and texts, they will always maintain their sharpness. This is because they are vectors.
With Photoshop, retaining the sharpness of zoomed images will require a higher number of DPI.
The T-shirt type you are using and working on will also determine the program to use.