Reddit organises itself into subreddits — independently moderated communities, each with its own rules, culture, and tolerance for self-promotion. Running one is mostly moderation work, and most of the decisions that constrain you later are made in the first ten minutes.
Creating a community is the end-to-end walkthrough: the account age and karma prerequisites, the fields set at creation (the r/ handle is permanent — the display name is not), the public/restricted/private choice, and the Mod Tools configuration worth doing before launch, including an initial rule set and automated moderation.