You need to take a screen printing class. Look around at craft stores.
1. No. You can not use window screen. You use silk or a polyester fabric that you stretch over a frame for this purpose. There are frames used specifically for this purpose that have a groove for inserting the fabric with a gasket and tool for this.
2. Any clear film can be used, including plastic used for overhead transparencies in school. I used to create my artwork on the computer and then print it on the acetate sheets. It worked rather well.
3/4. You coat the silk with the emulsion in a low light area and let it dry in the dark. Then you place the transparency, with the image in black over the dried emulsion. Expose it to light and the emulsion will "set" into the areas that are not covered by black in the transparency. Basically, it works the same way photographic film works. When exposed to the light, the emulsion hardens.
5. When the emulsion is "set" you rinse the screen and the emulsion will wash away from the parts where the transparency was black. Any pin holes or spots where the emulsion didn't set properly can be touched up with a liquid called resist. It's just another little thing that people who do this a lot use. It saves you a lot of extra work.
6. Then, when it is dry, you put your ink or paint on the top of the screen and pull it down over the paper, shirt, poster, whatever, with a rubber squeege. The ink will pass through the open areas and onto whatever you are printing. You should do several test prints before putting the design on shirts.
If you need more info, you can contact me. I actually paid a big part of my tuition in college by printing t-shirts for clubs and events on campus.
I bought most of my supplies through Pearl Paint or Friedman's in NYC. However, almost any good art supply store has the materials. Places like Michael's and A.C. Moore's should have what you need.
You might want to buy a kit to practice first. Then you can buy refill supplies and add-ons as you master the process.
Hope this at least gives you somewhere to begin. Good luck.