Film positives, or transparencies, are what you use to copy your image onto the screen. These thick, waterproof transparencies are essentially stencils that you put on top of your screens in the exposure unit to transfer the image. Just make sure it’s a high-res file in the darkest black. (If you want to get specific, at least a 300-dpi, spot color image works best.) If you’re foraying into screen printing for the first time, you might want to try out one color initially. If you’re printing in more than one color, you’ll need to make a separate transparency for each color.
The image elements for each color also need to be separated out. For instance, if you are printing in red and black, you’ll need a stencil for parts of your image that are red, and another stencil for the parts that will be in black. As for thickness, a 4-mil transparency should suffice. Of course, before purchasing, make sure the kind you’re buying is compatible with your printer.