Food is generally the easiest reinforcer to start with. Later on, you can incorporate games your dog likes.
Think of the clicker like a camera with which you “take a picture” of behaviors you like, as your dog offers them. If you are clicking sits, click as your dog’s butt hits the floor, then deliver a treat. If your click is late, you missed the picture – click as your dog sits or lies down, not after.
When clicker training, avoid adding a cue until the dog is performing the behavior reliably. You want a nice steady rhythm of behavior, click, treat; behavior, click, treat. As soon as your dog eats the treat, he offers another sit.
At this point, you may begin adding the cue. Say the word (sit) as your dog’s butt lowers toward the ground, then click when he sits. Toss the treat away so he has to get up to eat it, this sets him up for another sit. If he offers a hesitant lowering of his butt, click anyway – he is trying to make the association between the word and the behavior.
Once he is responding to the cue reliably, start proofing for fluency so that your dog will respond to the cue reliably in a number of different environments, even at a distance.