Headers are useful for navigation by all viewers, but particularly by those using screen readers.
Choose the style header you want to use, and you'll note that it will automatically generate a hierarchy of headers for your document using that style.
For more information about using the Styles gallery, check out Microsoft's Add a Heading documentation.
How you add Alt Text will depend upon the version of Office you are using. In general, click on the image (graph, etc.) and choose Format.
You may also search for Accessible Templates on Office.com.
For an excellent overview of how Microsoft updated existing templates they have included in their accessible template selection, visit Get Accessible Templates for Office, and go to the "What makes these templates accessible?" section.
This will generate a report in the document that shows potential accessibility issues.
On a PC, click File > Info > Check for Issues > Check Accessibility
On a Mac, check the "Review" tab.