Here are some references that will help you:

Understanding styles | ShaunaKelly.com
Understanding Styles in Microsoft Word
Working with Styles
Numbered Headings
Why use Microsoft Word’s built-in heading styles? | ShaunaKelly.com

Of course, you don't need to read all at once but, they are extremely good references and if you follow the instructions and work with them you will gain a good grasp of MSWord.

HTHs...