For most things you can refer to these general instructions that cover most of what you need to know:
You can also refer to already-published pages and posts for a model. Open up a page or post that’s similar to the one you want to create and check how images, texts and other data have been added.
A few extra notes for your website particularly:
The easiest way to edit the excerpts for the homepage sections is to be logged in first and then to use the “Edit” links you’ll see in each section. They’ll take you to the main page for that section and you’ll find the homepage text in the “Excerpt” field over on the right.
To add articles to the media section, please use the custom post-type “Articles” in the black sidebar.
Be sure to fill out all the fields (use an existing article as a model).
Other than the media section, the page content is all available in “Pages”.
If you have any trouble, please re-read the Basic WordPress instructions to be sure the answer is not there. If you are still stumped, feel free to request help by email: email@example.com.
Also, you’ll find some resources about maintaining ongoing website health here.