There are a few ways to send messages on this webforum:
1.) The Message Boards
Follow this Link then choose from the title specific boards, general discussion, off topic discussion, or feedback boards to post your messages.
I suggest that if you want to just test out how to post, that you do it here: Link
You post messages by either clicking a thread and hitting the "post reply" link in the upper right hand corner of a post, or if you want to create your own topic, by clicking the "new topic" button in the further upper right hand corner. Regardless of which option you select, from there you just enter your text and hit the "submit" button to publish your post.
You've already figured out how to post a blog it looks like, and replying to blogs is a bit simpler than that of the message boards, if you want to reply to a blog all you have to do is go to the blog of your choice and enter your message in the comment box under the blog and hitting the "submit" button.
3.) Private Messages
If you want to send a direct message to a certain member, then Private Messages (PMs) are the way to go.
To send a PM, first click on the person's profile you want to send the message, then click the "new private message" link. From there you just enter your message header and message content and hit that same "submit" button to send the message.
If you want the check your private messages, you just click the "you have __ private messages" link in the upper right hand corner of your dashboard. (if you have a new PM, then a small "NEW" sign will be blinking)
Hope this helps!
Also, here's some threads that may help you out as well: Link Link