Thats a question for a pro. Different by state.
If the dog is pedigree, register with AKC. The term register usually the dog is purebred/pedigree and you wisn to show the dog in dog shows. I think you mean getting the dog microchipped. If you resued from ddfl, go there. And you can microchip at Alameda East Vet Hospital. Most vet hospitals and anim shelters have microchip scanners. If your dog goes awol, the dog can be scanned and a read out comes up with its name your name and phone.
Is the dog from Dumb Friends league? If so, give them a call and ask for their protocols on transferring ownership. Is this dog vaccinated currently? Do you have paperwork or know the previous vet? Is this dog a puppy? About 2 months? Then you need shots at 8, 12, 16 weeks. Find a vet to start this process. Is this dog older than 2 months? Then will need 2 sets of shots. Please reduce contact with all other dogs until fully vaccinated. Finding a vet is your next step, reduce contact with outside dogs in the mean time. If you have other dogs, they should be up to date on vaccines as well.
Take your pet to a Vet and they will give the dog all needed shots and can also check the health of the dog and chip the dog if desired.
It depends on what state and city you live in. They all have different requirements depending on where you live. A Google search will usually tell you or you can contact the county you live in and they will be able to direct you on your next steps
If the is a pedigree animal and you have the information you go to the AKA for "registration". If you are just looking for a local registration then call Animal Control and they will tell you what to do for your municipal area. In some areas it just requires shots and your tags are considered to be the registration, in others you will need shots and to pay a fee.
Best bet is take it to a vet get the chip # & persons information who originally sold/gave the dog away, then start tracking backwards. You'll need good records in order to register of it's a purebred.
Thanks! Your answer is awaiting moderation.