How do I delete everything in Redis?

With redis-cli:

• FLUSHDB – Deletes all keys from the connection's current database.
• FLUSHALL – Deletes all keys from all databases.

For example, in your shell:

redis-cli flushall

Heads up that FLUSHALL may be overkill. FLUSHDB is the one to flush a database only. FLUSHALL will wipe out the entire server. As in every database on the server. Since the question was about flushing a database I think this is an important enough distinction to merit a separate answer.

Answers so far are absolutely correct; they delete all keys.

However, if you also want to delete all Lua scripts from the Redis instance, you should follow it by:

SCRIPT FLUSH

The OP asks two questions; this completes the second question (everything wiped).