The CSS Box Model.

So you want to do some design in css, but you don't know how to place your content in a way that it is accessible and scalable, well that's where The Box Model comes in. 
The Box Model sets the perimeters of the container which will be holding your information/content. The three parts of the container that you can manipulate are the Margin, Border, and Padding.  All of these elements together make up the total size of the container.
An easy formula to remember when formulating the total size of a container is this:

Here is an example of what the code for the box model would look like.

I hope this tutorial/explanation was helpful and good luck on further css adventures!