Here’s the link to the website where you can sign up for a free WordPress (WordPress is a type of blogging platform) notebook: http://onsnetwork.org/
On the right side of that page is a “button” that says “Start a New Notebook”. Click on that “button” and that will initiate the sign up procedure for Open Notebook Science and launch your new notebook. It may be a bit overwhelming at first, as there are loads of customization options (should you choose to go down that rabbit hole). However, you can keep things simply by leaving everything set up with the default settings and just try adding a new Post and see how it goes.
Here are links to our notebooks (you can see how different they look):
Additionally, Jake uses Google’s Blogger for his notebook. Check it out here: http://heareresearch.blogspot.com/
Finally, our undergrads use Wikispaces (click on Katie, Grace, or Jonathon’s notebooks to see examples): http://genefish.wikispaces.com/
(thanks to Sam for actually answering the question)