Look like you have figured out how to post a new message and have also found the Reply button.
The next most handy thing is the FIND button/menu, which allows you to search for messages on specific subjects or to locate new meesages since your least visit to the forum.
You may also want to register for the Classified ad section and RV Park review section - they require separate logins. You can probably register for them with the same name you use here, but it still needs to be done. Links to them are across the top of the page and on the irv2.com home page.
There is a separate Tech Support & Help section of the forums if you can't figure something out.