There is no certification in the US. You have to be disabled, the dog has to have individually trained tasks to mitigate your disability, and the dog has to be well behaved in public. Then it's legally an SD.
You do not need to start with a puppy. I actually prefer not to, because right now especially I'm in no position to raise a puppy. I got Logan at just over a year old. His breeder socialized the
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out of him and started his training. He had some pretty solid obedience on him when I got him, which was great. I've had him for 5 months, and I expect to have him close to working full time by February at the earliest, but my actual goal is May/June of next year to be 100% full time. So that would be 1 year. But, I started with a dog that already had a good deal of training on him (he passed his Canine Good Citizen and Therapy Dog International tests 3 weeks after I brought him home, but I decided against sending off his TDI test to actually become a therapy dog). Logan also just got his Rally Novice title. Gavroche got his this spring, after I had him for 1 1/2 years. I only had Logan for 4 months when he got his title. Generally, it takes 18 - 24 months to train a service dog, starting from scratch.
It's always a good idea to have another trainer, if nothing else than to have a second opinion on things. I have a good deal of trainig experience, but I still work with both my rally trainer and my dad, who has been training dogs for decades and has titled many, many dogs. My dad also has a working dog training background - including service dogs, sentry dogs, schutzhund, obedience, tracking, personal protection, SAR, he was a dog handler in the army, he trained for the Perdy Prison program - he's done a lot of different things (that being said, no service dog should have any personal protection/bitework of any kind, as the dog is then considered a weapon and it's not really a reasonable accommodation anymore). Between them and the people here, I'm always re-evalutating, adjusting, and trying new things. I also had a practice dog (Gavroche) to train all on my own before I ever got Logan. I didn't really intend for Gavroche to be my "experiement" dog, but that's kind of how it works out with your first dog. I've trained dogs before, sure, but never all on my own. Gavroche turned out pretty well, but of course I made mistakes. He's also just not SD material.
A service dog can be any breed - it doesn't need to be purebred. But often it's easier to find a suitable candidate from a good breeder. I suggest you have someone who has experience selecting an SD candidate help you choose a dog. Just going to a shelter and picking a dog is setting yourself up for failure. It may work out, but odds are it won't. The breeder I got Logan from actually had 2 dogs left from his litter, and I had my eye on the other one (Amos, a male sable smooth collie). But between the breeder (who also happens to be a dog trainer) and another mutual friend of ours (who has a very nice Borzoi SD) it was decided that Logan was actually the better candidate. And when I went and met them, I knew they were absolutely right. This breeder also does Early Neurological Stimulation. The difference between Gavroche and Logan when they're at a dog show or another stressful place is night and day. Logan is pretty much the same dog he always is. Gavroche almost shuts down and gets stressed. I got Gavroche at about a year old, too, but he was a stray and I have NO idea what his history is.
Training a service dog is different from just training a dog. If you've never done it before, definitely get help from someone who can point out errors before they ruin the dog, give suggestions, bounce ideas off of, etc. If I didn't have my dad, my rally trainer, and everyone here, there's no way I'd be comfortable owner training right now.