There are many types of collars to choose from depending on your dog breed, function and needs.
Purchase one that has the proper width for your dog size. Small dogs should have small lightweight ones. They should be soft but well fitting. Most are nylon or leather, nylon being popular. Rolled leather however is often preferred for long-haired breeds.
Buckle collars are adjustable and flat or rolled. Most of them have a breakaway buckle so dog can get free if necessary. They have rings for identification tags.
Quick release collars are quick to take off and put on because they have plastic fastenings. They come in flat nylon and to are adjustable.
Training collars are also called choke chains and are used only for training. Because they don't break away be with your dog when he has it on or he could choke or get killed if caught on something. These are used most often for strong-willed dogs or those who don't respond to training with regular collars. Dont' use them with dogs with delicate tracheas. Don't use these collars for puppies!
A good guideline for a proper fitting collar is to place two fingers between dog's neck and collar and if it feels snug it's a good fit. You don't want the it so loose that it slips over dog's head.
The collar should have identification tags on it with your name, address and phone number. Never include a dog's name on his tag to allow a thief to call him by name.
Remove them and check regularly to remove dirt, hair and to see if stiff. Also check to see if it's still fitting properly.
You need a leash that fits your dog breed and size. Nylon are the sturdiest and lightest which are good for all breeds. For a young puppy you should have a 4-6' nylon leash. An older dog a 6' leather or strong cloth leash.
A retractable leader which is a nylon leash incased in plastic casing that reels in automatically is good for walking and traveling. They come in different lengths. The best type is the one-button system which stops a dog going beyond a certain distance.