My GSD puppy keeps barking at people when I am walking him down the road, he also barks at passing cars how can I stop him doing both of these things?

Ohh German Shepherds are very loud dogs. Get his attention, tap his neck, stop him before he starts. Teach him the command "Settle" tap his neck as you tell him to settle, when he stops, give a treat or continue the walk. Hope I was help. :)

Thnak you will try that. Even at only 3 monthes old it is still very intimidating!

Teaching your dog to focus is a great tool.

Start from home and get your dog to sit while you are holding a treat.
After he sits get him to watch you while repeating your chosen command (I use "watching") after he has been staring at you for around 5 seconds give him the treat.
Increase the amount of time he has to focus on you each time.

Once you have perfected that get a family member to walk behind the dog while you are giving the "watching" command.
If the dog look away, start again.

And finally, once you have perfected that you can go out onto the street to do the same thing.
Every time you see a car or person approaching get him to sit and watch you, eventually he will learn to ignore the distractions around him all the time.