First read all you can find online about pcb layouts in general. Then read about the software you are using.
You will have to contact your anticipated fab house. Each will have min trace widths, thicknesses, drill sizes, etc. They should also have guidelines on parts placements and margins, of course depends on parts. I use futurlec.com, they are really helpful, and can do board stuffing (but I disclaim, I have not used this part of their service).