Blog!
Blogging is super-easy.
To create a blog, you need to be in your directory (get there by clicking on your login-name in the navbar just below the tabs at the top of the screen).
There is a drop-down menu in a bar up at the top of the center section of the page that says "add new item". The first option is "blog"; select this option to create a new blog.
When you create a blog, you will be asked to list possible categories; whatever topics you choose will become part of a menu you choose from when you create a blog entry.
Once you have created a blog, you can post a blog entry, or create blog folders to organize your blog documents.
You can also add files, images, and links to your blog.
If you want to change something about your blog after you've initially created it, click on the tab just above the main content area that says "edit".
If you have questions about how plone works, check out the plone tutorials.