The forums are very vast, over the years it has become quite the collection of magickal resources and discussions! I completely empathize that it can be overwhelming at first. That being said, you’re already off to a great start! Sharing your introduction is, in my opinion, the best way to begin 
From here, all roads are open to you! I would warmly encourage you to browse through the main page of the forum and click on any discussions that interest you. If there is something in particular you’re learning about, the handy Search Tool will help you locate it quickly.
Yay! The challenges are weekly events - a new one begins every Wednesday. They can be found (usually) on the main page of the forum. If you can’t see it there, no worries - stop by the Activities category and you’ll be able to see it in there.
(Speaking of which, here is this week’s challenge if you haven’t seen it already - it will remain open until tomorrow! Weekly Witchy CHALLENGE - Intuition )
I would recommend reading through the first post as it has an introduction to the theme and some suggested ways to explore it, but the best part of the challenges is (in my opinion) reading everyone’s entries - the coven is so creative and inspiring! There are lots of great ideas shared in the entries 
If a theme doesn’t call to you, no worries! There will be a brand new challenge with a new theme every Wednesday. Know that you are warmly invited to join in if you’d like! 
Blessed be, Niyke!