The mechanics of posting in this forum should become pretty intuitive with a little practice.
To Begin a New Topic
Click "+New Topic" at the top right of the forum page. Enter a thread title with at least 15 characters (A ton of extra spaces won't work as filler). This is not hard to do and just means you need to spend more than a millisecond thinking of a halfway decent title. If you knew how many threads titled "Question" or "Newb Needs Help" we've dealt with, you'd laugh, and then cry.
The topic text box has a 20-character minimum. Again, this is not hard to do. Any question that's worth asking will/should be more than 20 characters. "How to lose fat now?" would hit the minimum, and that's a ridiculous way to phrase a question.
Click "Create Topic" under the post and you're good to go.
Couple of options here. First, a general reply by clicking "Reply" at the very bottom of the page. This will bring up a basic message box where you can type a reply. Again, there's a 20 character minimum, but any reply that's worth sharing will be at least a few words. Not a big deal. "Yep", "lol no", or "You're wrong" don't exactly contribute to a quality conversation. Hit Reply and it's up.
You can also reply to a person's post by clicking the Reply button directly underneath that post, which will direct the reply to them. However, this isn't the same as tagging them in your reply. You can type @ before their username (autofill suggestions will come up so don't stress over spelling) which will send that person a message they're being directly replied to.
Last, and probably least frequent, option is to Reply as a Linked Topic. This basically lets you create an entirely new topic that links back to the original topic in its first post. Useful for catching yourself before you hijack a thread. If a thread's talking about how to determine openers for your first powerlifting meet, and you're reading it but want to ask about deloading for your first meet, that'd be a good use of 'Reply as a Linked Topic'.
To do this, hover to the right of a given post just next to the post date, then click "Reply as Linked Topic." This will bring up a text screen similar to starting a new topic, with the default text "Continuing the discussion from [a hyperlink to the first thread]. Select the most appropriate new forum section (almost always the current forum you're in), enter a thread title and your own text, and then "Create Topic."
To Quote in a Reply
Replies, even direct replies, don't automatically quote text. If you want to quote specific text from a previous post when typing a reply, highlight the text and click "quote reply". The quoted text and username will be inserted into your reply.
You can also skip a half-step and just highlight/quote a section of text. Clicking "Quote Reply" will automatically open a new reply with that text. Very useful shortcut.