One of the most important things to teach a service dog, or any dog for that matter is how to walk politely on leash. Sometimes called 'loose leash walking', this habit is crucial. It prevents falls, and keeps your dog in reach if you need them, without pulling and forging.
There are lots of videos on this topic on Youtube, but the majority of them consist of some kind of aversive treatment, such as head halters, prong collars, or harsh corrections. Why not teach your pup what you do want, instead of making them guess?
The general scientifically proven method to teach a dog to walk without pulling is to make being beside you the best place ever.
How do you do that? By allowing them to be rewarded when they assume the position, of course!
I've used the videos below, or used a spatula or wooden spoon with peanut butter or spray cheese on it to lure the puppy into place beside me, then eventually take a step, luring the dog forward.
Do this beside a wall or a couch to prevent them swinging out and going in front of you.
These three videos are where I go to teach my pups how to walk nicely beside me. It takes a while to go through the technique, but in the end, you have a well behaved dog that sticks by your side, matching your every step.
Start with short sessions - this type of training can tire a dog out, especially when they're young. Five minutes is the maximum amount of time, per day, that they can retain information from.
Then go on to play with a flirt pole, throw a ball around, or just play tug. Going for a nap is good right after this kind of training. Then the information has time to sink in and be retained.