On the Home page each Board is shown in bold type on the left, next to the folder icon. When you hover your mouse over the name of the Board it changes colour. The latest entry on that Board (called a Thread) is shown on the right-hand side. You can click on the title of that Thread to read it or to reply to it. (Tip = do not click on the name of the poster, it will take you to their Profile setting).
Below the text of the post is a box like this one (click on the image to enlarge):
Just write what you want to say in response to the first post. If you don't want to add any other formatting or images or quote what has been written, just press the Quick Reply button at bottom left and it will be saved. If you want to do some of those things, and review what you've written, then press the Reply button at top right and you will get access to a Menu bar above your text with these tools:
If you click on the Help button in the main menu bar (at the top of the page of any screen) it will take you to the Proboards Help site and will give you detailed advice on how to use all the tools.
You can also send a message to the Administrator or the Moderators and they will try to help you with any problems you have.