Hi. Welcome to the forum!
You can post, like you have been in this thread, by hitting the reply button at the bottom of the posts. If you want to respond to an earlier post i.e. not the latest one) click the little reply arrow next to that post and it links back. You can also select text from someone else’s post and it’ll give you an option to quote, which opens up a reply and inserts the selected text.
Hopefully that makes sense but just shout if you need help or have questions.
As @Iron says, you can also send a private message to someone by clicking their avatar and hitting the ‘Message’ button.