For those that don't know, a clicker is a small plastic and metal hand held device that makes a "click" noise. When the dog offers you the behaviour you want you click and give a treat. Why not just give the treat? Because dog training is all about timing, and if your timing is off you will reward the dog for the wrong thing. What the clicker allows you to do is be slow or clumsy with the treats. The dog learns that the click means "yes, THAT is what I want" so even if you have to rummage around for a couple of seconds to find the treat in your pocket the dog knows which behaviour it was that got him the treat. Therefore, in order to begin using clicker training you must teach your dog the simple equation, click = treat. The following video demonstrates how to teach your dog this. I apologize because Summit had already learned the clicker before I took this video so he catches on really quick. In the beginning you may need to put a treat on the object and click as the dog takes the treat himself. After doing this a couple of times he should go back to look for a treat, and when he does you click and treat. When your dog looks for his treat when he hears the click you know he's put two and two together.
Targetting is also very useful, and it's a good and easy way to continue with clicker training. Pick something you'd like your dog to target. In my case it was a toy to try to teach Summit to retrieve. In this video I ask him to target my hand to show you the steps because he now retrieves toys instead of targetting them. I started by putting a treat in my hand. When he sniffs my hand to look for the treat I click and give it to him. After doing that a couple of times I stop putting the treat in my hand, but he still comes looking for it. Click and treat.
Finally, once you see the light is on you can start introducing a verbal command. When do you know the "light is on"? Notice how he touches my hand and immediately looks to my other hand where he knows the treat is coming from. He gets it.
EDIT: I wrote this entry in a bit of a rush. I should also mention that you can teach your dog a "verbal click". What this does is allows you to clicker train even if you forget your clicker. Pick a crisp, short word. Most people use "Yes!", for whatever reason I started using "Good" so that's what I continue to use. You will often hear me saying this whether I am using a clicker or not. The problem initially with using a word is that it doesn't sound identical every time. The great merit of the clicker is that the sound is always the same. You will note in my videos when I am "clicking" using my word, that my intonation is different, pitch is different, etc. Summit knows this well enough now that he looks for his treat when he hears me say "good", but in the beginning the clicker is more consistent.
It is also important to note that if you click you MUST treat. I think in the last video the very first time I ask him I make a mistake and click even though he did it wrong. I still gave him a treat. Sometimes you click too early because you're excited, or because you've got a clicker and treats in one hand and a leash in the other you're not too coordinated and you click by accident. If you clicked you have to give a treat, even if it was an accident and the dog didn't give you the behaviour you wanted. By extension it also means don't click if you have no treats! I always carry a treat bag so I've always got treats, but if you don't then you shouldn't click.