How to go about getting images screen printed on shirts?

I'm starting my own clothing line and I want images that I take with my Nikon and edit on the computer to be screen printed on a bulk order of shirts. I did a bit of research and it seems that I have to create the images on Photoshop using certain color methods etc. However I don't have photoshop and am not familiar with using it if I had access to it. Is there any other way I can print out the image I want and take it to a screen printing shop to make the shirts? Please let me know!

(images or text like Nikkilipstick.com)

If its a image used with your nikon take in your camera tell them you want that screen printed they will use what is called 4 color process and give it to you depending on where you live a lot of print shops won't do it unless its a large amount but go around and ask and usually on photoshop you use cmyk with a good amount of dpi but that is for printing flyers business cards etc Mostly for Designing shirts you use Adobe illustrator, Corel Draw or any vector program: D