Css makes it so much easier to manage your website and change layouts more frequently. First you are going to need to make a page called "style.css" once that page is made you need an actual css code. If you don't know how to make one, I have some. (Click Here)
Alrighty once your style.css page is done. Take this code and put your site url in.
<link rel="stylesheet" type="text/css" href="http://www.YOURWEBSITE.com/style.css">
That code there ^ Needs to be pasted onto each and every one of your pages. But make sure you haven't added any colours or fonts (so its just a white background with black text at the moment)
Once all your style.css page is uploaded along with all your pages, your site should now have the new css on it.
When you want to change your layout, you only have to edit your css on the "style.css" page.
If this tutorial helped you, please link back. =D