You should look up how to get the data from the website, and you should also make sure that you understand html. You also need a simple way to handle html.
- Get the page (use the example in the question): Read data from webpage
- A bit about html: http://www.w3schools.com/html/default.asp
- Handle html: http://jsoup.org/cookbook/extracting-data/dom-navigation
To login you should do a post request with the login information to the standard login page, then you keep the cookie that were generated and pass it with your other requests.
- Little about handle cookies: Java: Handling cookies when logging in with POST
Some things you also might want to think about:
- Linear or branched view of posts in the forum?
- Should you get a message if someone post a new post?
- A own search function?
- Signature?