I just type this response in the box underneath and hit post reply. I'm only new but this is a great sight I'm sure others much more advanced will give you more info. Welcome aboard
Hi @bevellar and welcome to the forum!
If you want to send a Private Message to someone, click on their name (in red under avatar) and you will find a box to send a private message (PM) to them. Or you can put their username in a post with an @ before it, eg @bevellar, and they will get a notification that you have tagged them in an open reply on the thread.
If you want to reply to any thread there is a box below the last post where you can reply, or if you want to reply to a specific post there is a small box saying 'Reply' in the bottom right corner of the post which you click on and that will open a box to reply in.
The more you use the forum the easier it gets.
There are some videos that will help you with posting and PMs. Regarding PMs, you will have to make 16 posts before you can use this facility. This has recently been introduced to stop spammers getting in touch with members.