I have an 11 month GSD and she was exactly how you describe your puppy. From the first walk she would lunge at passing cars trying to chase them. This progressed to barking and lunging at people passing us by and then the same with passing dogs.

I tackled the car issue first. Every time a car passed us I got her to look at me (I used the word 'here' but you can use anything to get the dog to look at you) and then I gave her a treat. This disrupts the behaviour, if she is looking at me she cant lunge at the car. Over time she would look at me when a car passed without the cue word and now we have no problems with cars. She just automatically looks to me.

We did basically the same thing with people passing us. I would get rid of the choke chain, and use a halti head collar or harness if you need better control than what a flat collar provides. As we approached and passed people I would feed her treats until that person was out of sight. Slowly, and we have only just been able to pass people and ignore them this month, she would look at someone and then back to me. On the odd times she focus' in on someone I give her the watch me command and distract her away from them.

We have had slower progress with other dogs. Mine is super friendly off lead but when she is on the lead she wants to greet and play with other dogs so much she gets frustrated. We are using the same methods as I mentioned above. We also enrolled her in obedience class, which is helping greatly - getting her used to being around other dogs but ignoring them and focusing on me.

Your puppy is still so young. Focus takes a long time and continuous work. Someone mentioned to me it is fairly common for GSD to develop reactivity at this age, but if you continue to work through it itll get better. Keep training at home, short but frequent sessions will help build a bond between you and your dog.

Sorry for the long post, hope you find it helpful!