New content may be added to the site using tools that are part of the site software. If you have the needed privileges to do so, a "Create content" link will be visible below the "My account" link in the left sidebar under your username:

This will open a page that shows a list of the kinds of content that may be created. For example, to create a blog entry, click on the corresponding link:

This will take you to the content creation page for blog entries, which allows you to fill in fields such as the title of the blog and the body of text. The notes at the bottom of the "Body" field tell you which HTML tags you may use here. When you have finished setting the title and body, click the "Save" button at the bottom of the page to add it to the site.