Hiya and welcome! I am also new as of last week too! Don't panic, there is a lot of rules and guidelines on the home page to get through, also some new rules added (I just go through each board gradually every time I log on to get as much info as possible. It can be daunting to navigate round a new forum but trust me, you'll get there eventually!
If you have a look at the home page and the main areas like Showing Tab, Selling/Auction tabs etc you'll be able to see what threads about what content is and gradually pick up where you post threads about a certain subject etc.
I don't know if you've noticed, at the bottom of any page you're on, there is a 'Shoutbox' it's limited to 256 character's but if you have posted a horse for sale on the sales page but not got much traffic/views, then you could put a brief description in there about it for example, which the majority will see when they log on not long after you've posted it.
I still have a lot to get the hang of but with showing your horses, one tip that i've made sure I do, is read the description of the Rider and Horse attire needed etc, I have entered 4 horses into a jumping competition to just realize that my Beginner Unedited, meant the jump should be a certain height. (Makes sense now!) At first I thought it meant our photo's are judged by the editing or non editing skills.
I'll stop waffling now and am sure you'll be answering my questions lol