When you find the forum where you would like to post a messages, click on the forum name and it will take you to that forum group. The rest is easy. The page will look like this and you will then click on the "new topic" button and your new message page comes up waiting for you to fill it out and post it.
When you find a topic that you wish to reply to, it is also very easy, on the upper right top and bottom of the post are tabs, click on reply, fill out your message and post it.
It will all become very easy once you have used it a few times. Have fun. ;D