Well, if you want to be considered a reputable breeder you will start showing your dogs or competing in working events with them. We all need that outside perspective that comes from directly comparing our dogs to others' in a competitive environment. It's easy to sit back and be complacent, thinking "my dog is good enough", when in fact our kennel blindness has hidden our dogs' faults from us.
You will also join the German Shepherd Dog Club of America, http://www.gsdca.org/ your local German Shepherd Dog club and your local Kennel Club. These groups are excellent opportunities for education and networking, and you will have access to successful, long-time breeders and their knowledge.
Of course, you will be doing the health certifications recommended by the GSDCA. For example, hip and elbow certification by OFA, eye certification by CERF, etc.
And of course, you will breed for health, correct structure, temperament and breed type, ignoring passing fads and certainly not breeding for periperals like color (especially "rare" colors).
There's more, but a lot of that can be learned by the contacts you will make through the clubs and competition.
Read everything you can on dogs, German Shepherds, breeding, training, dog psychology, health, nutrition, and anything else that might be useful.
Responsible dog breeding is an ongoing education.
And because you love the breed so much, you will become involved in rescue as much as you are reasonably able, whether by fostering, transfering, doing home checks, referring, donating, whatever you can do to help the breed you love.
And of course, you are breeding strictly for the love of the breed, not to make any money.....