Too Long; Didn't Read
Reddit is a huge forum with a bunch of subforums called subreddits called subreddits. Each subreddit is a closed ecosystem with its system of rules, established practices, and most importantly, an audience and moderators. The first thing to remember is the karma system. Each comment or post can be rated up or down. The more positive ratings (called "upvotes" here) you get, the more roads will open up in front of you! You need a minimum of 150 karma points to get started on the platform = to be eligible to write in the most popular.