Empty CSS Template

I guess you could say I want to do this the easy way. I would like to change the style of forum using CSS but I don't know how to organize the script so that it works with the forums. So What I'm looking for is an empty CSS code template that all I need to do is fill in the values and be done. I know enough CSS to be able to do that much but my issue is I don't know how to organize the attributes or does the organization of the code not matter? I would love to know the organization so I can get started. Another question is what size, in pixels, should image icons be?

thank you that really helps