I have found the best way to desensitize a dog about his leash is to put the leash on him and do nothing. Don't take him for a walk or even go outside. do this over and over until he no longer reacts when putting the leash on. then maybe short walks in the house and then very short walks in your yard. eventually you will be able to go on longer walks. reward him for each achievment.

Immediately, the sooner the better. crate training is one of the basic tools you can use to help with structure and makes potty training your puppy much easier.