Competitive Obedience is a sport, and has been such since the early fifties. You can learn to compete in obedience either by joining a club or learning on a 121 basis with an instructor.
When ready to commence competition most people start with companion shows where you just show up and enter on the day, they normally hold classes especially for those just beginning to compete. From there you can progress through various competency levels.
Limit, Open and Championship Shows are run ‘by the Kennel Club’ and ‘Under Kennel Club Rules’. Each Show has closing date for entries a few weeks before the actual event, which means you must pre-book and pre-pay your fee.