How do I delete a subreddit I created?

You can't, subreddits are a community resource. If you no longer have a use for a subreddit you created, then update the description with something like "I have no use for this subreddit. Please PM me if you'd like to use it." If someone contacts you, then make them a moderator, and remove yourself.
You can also make a post over in /r/adoptareddit.

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