
Congratulations on your puppy!
To be able to show her you need to make sure she's Kennel Club registered. Hopefully her breeder shows her dogs and will be able to advise you, but if she doesn't (and even if she does!) it'd be well worth joining one of the Cavalier breed clubs; details of these can be found at the bottom of the
breed standard.
It'd also be sensible to take her to ringcraft classes as soon as she's finished her injections; this is the best way for both you and your puppy to learn ring procedure and how to present your dog to her best advantage. Your vet might know of ringcraft classes in your area.
This page from the Kennel Club website gives lots of useful advice too. :-)