Different mesh counts for different jobs. The higher the mesh count, the finer the detail or line you can hold with your screen and print in your design. After you have your screen you will need to apply emulsion onto its surface. Emulsion is a photosensitive solution that reacts to UV light. It reacts in the same way that film reacts to light when you take a picture.
Emulsions can be in liquid form or in capillary film form.
Most emulsion manufacturers suggest three coats squeegee side and two coats print side. If you are doing print runs under 144 items, my way works very well. It is my belief that emulsion makers are out to sell more emulsion. After the screens are coated they need to be put horizontal in either a yellow lighted or no light room to dry. It is a good idea to run a small fan at low speed close to the screens to aid in the drying process.