You are going to get some abuse for that question, there is a simple search function that will really help you on here!!
But for now, here are the basics.
Big lever, gear stick, little lever, difflock and high/low ratio. With me so far??
To engage low ratio, push said little lever forwards, do not do this at any speed 3-4 mph will be ok but better stopped. If you have an auto then put it in neutral or it won't go in to low box.
If you need difflock, then push said little lever sideways away from you and that locks a centre differential, so locking your front and rear axles together so to speak! (power equally front and rear) There is a light on the dash that should glow, but sometimes they dont. DO NOT engage difflock on hard surfaces, you will break something eventually!
To disengage simply pull little lever back across towards you and light should go out. If it doesn't then reverse back a couple of feet and that will sort it.