I took a few printmaking courses while I was in college but I forgot most of what I learned :-P. I looked up the screen printing process on the 'net and came up with the site listed as the source below.
Here's a quote from the problem solving section:
"Problem: Too much or all Photo Emulsion washed out of screen after exposure.
Improper mixing of emulsion and sensitizer.
Tracing paper or film not sufficiently transparent.
Washout temperature was too high.
Screen not dry before coating.
Screen not dry before exposure."
If a photo type picture you can use Photoshop to convert it to CMYK. Other types of graphic you need to convert it to solid spot colours - Illustrator the best choice for this.
Creating the separations is done at print stage, any desktop publishing software will have a setting for this in its Print dialog, printer needs to be a postscript device.