Good luck with that Corinna, I have been trying to get my dog Zoe to stop chasing discs. Zoe doesn't retrieve them though, just slobbers all over them to let me know she found it first.
Most service dogs begin training at about 16-18 months of age, so a puppy may have a problem learning difficult tasks.
To train your dog I would first get the dogs favorite treat or toy. Walk the dog to the disc make it sit and speak and then give it the reward. It sometimes takes at least 50-60 reps, before it catches on.