i have a Toyota Rav4 2006 4WD but i don't know how to engage and disengage 4WD and how it works

Try you owners manual, http://www.toyota.com/owners/resources/owners- manuals/rav4/2006

The car makes the decision on what percentage of power goes to the front or rear wheels. There is a computer that detects slippage and assigns power accordingly. You do not have to do anything. If you have 4WD button on your dash, that is so the driver can put the car into 4WD at speeds under 25 mph. But you really do not have to use that button as the car computer does a fine job without it. I have driven a RAV4 with this system for years in many snow storms and find it to be an excellent system that can get through snowy and icy conditions as long as the snow is not so deep as to reach the body of the car.