Like Nicky09 explained, you have to get the dog to understand the association between the clicker and reward fisrt in order to be able to 'mark' the desired action the split second it happens. This is called conditioning. Here's a video to show you how to do it YouTube - Dogmantics- Dog Training Television- Episode 1

Once the conditioning is in place you can start to associate it to various actions such as 'sit' etc... try this video from Karen Pryor a pioneer of clicker training YouTube - An Introduction to Clicker Training

If you want to read up on how far you can go with clicker training (over the coming years not just months!) try
Clicker Training for Dogs by Karen Pryor
or
Superdog by Mary Ray

Have fun!

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