Why do you want to start at all?
Is she an impeccable example of her breed? Have you shown her? Has she had all neccessary health checks? Is she KC registered? Can you get in touch with her breeder?
Are you prepared to spend a lot of money? A good breeder never profits. Are you aware of all the risks involved – that your bitch could need a caesarian, or you could even lose her? Are you prepared to hand raise a whole litter of puppies in case something goes wrong? Are you aware of the legalities now concerning tail docking? Are you 100% confident you can find excellent, responsible homes for each and every one of her puppies? Are you prepared to take any one of those puppies back at any time in the next 14 or so years if their owner can no longer keep them?
Breeding is certainly not something to take lightly, and in my opinion best left to the experts. Bitches are much better off speyed. If you are thinking about breeding seriously and not just a ‘one-off’, if the answer to all the above questions is yes, then the next step would be to contact your girls breeder for support & advice. Boxers have a lot of health issues that certainly don’t need more irresponsible breeding.
Just my opinion.