Any pup over 8 weeks of age should be ready for a collar and lead, in fact, a lightweight nylon buckle or Quick-Klip collar that fits correctly should be your first purchase. It is best to wait until after your pup has become used to his collar before you attempt to introduce the lead. Leave the collar on as much as possible until your pup gets used to it. If he is in a cage or crate, remove the collar to prevent snagging. Do not use a choke chain collar on young puppies.
When choosing a lead, you must keep several things in mind. If you have a smaller dog that does not pull too hard, purchase a nylon or cotton lead. A chain or leather collar is best if you have a large breed or one that has a tendency to pull.
The lead length is totally up to you. If you are going to do obedience work, purchase a 6-foot lead - it is required in most classes. For just tooling around town with your pooch, a 4-foot lead will do. It will keep your dog near you and give you plenty of control. Retractable leads are also an option, but are best purchased once your pup is used to a regular lead and has had some control training.
Repeat this step every day for about a week and every day you should see your pup becoming more confident and yourself having a little more control.
Following these tips will go a long way towards making life with your dog more fun. A leash-trained dog not only puts you in control, but also gains respect from pedestrians you encounter on your walks.