Photoshop is the most common software used for screen printing. Photoshop is a tremendous tool in the screen printing process. With Photoshop you can create high resolution artwork, digital mock ups or proofs for your t-shirts and print film positives. I personally learned most of my Photoshop skills from Adobe Photoshop Classroom in a Book. Most screen printers output vector designs from Adobe Illustrator, but i personally use Adobe Photoshop to output all our shop film positives. I may create the design in Adobe Illustrator but I always save the file as a .png file and open in Photoshop and separate the colors in the channels panel with the color picker tool and print the separations using a RIP software.
Ultra Seps is an awesome plug-in for Adobe Photoshop that will help save to creating your color separation in Photoshop. We use UltraSeps everyday to separate our art to output film positives. Visit www.UltraSeps.com to download a free trial.
We use AccuRIP software- which can be found at www.solutionsforscreenprinters.com
Photoshop can also be used to help repair images supplied by customers. Many times you may be supplied low resolution .jpeg files from customers that will need to be "touched up". Using Photoshop you may be able to improve the overall file enough to print film positives and screen print the design. Otherwise you may end up needing to redraw the jpeg image or "vectorize" it in a vector software such as Adobe Illustrator or Corel Draw.