You need to plan carefully first what you need in terms of layout, whether sections are going to be similar, whether there is a lot of sharing or not. And the amount of formatted container your foresee.
It naturally occurs to you that if you can centralise the design on some CSS commands, you will benefit from being able to change looks at once as they are all linked to a common source.
If you are going to have several pages, you will be thinking of having the CSS even on a different page and link to it from others.
Also, you need to know that divs and spans work in a different way than other HTML elements. You will not be able to obtain effects that you would have obtained with a table.
In other words, what you need to do is go to the w3 website and read everything about CSS. Expecting to get an answer in 3 lines about such a huge subject will not help you much.
Alvaro