Click here to see a breakdown of the sub-forums, you can "follow" any that you think would be helpful or interesting to you: https://www.reef2reef.com/forums/
At the top of that and most sub-forums, you'll see "stickied" threads, these are threads that have so much useful information that the site admins keep it permanently in place at the top of the sub-forum. These stickied threads would be a great place to start learning about reefing.
And I'd suggest poking around the Member's Tanks sub-forum. Here, you'll see all kinds of examples of how people setup and maintain their tanks. You can often find someone who has the same tank that you're building: https://www.reef2reef.com/categories/members-aquarium-discussion.321/