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.