You start at the top and work your way down. When you arrive at the framework (that which supports the deck and railing) it can become quite precarious if you are inexperienced with the work. A much safer way is to rent scaffolding to work on. This way you will be much safer even though it will probably take you longer to errect the scafolding the to tear down the deck.
Tools: large crowbar, small sledge hammer, standard hammer, sweat and more sweat.
HAVE A GOOD TIME!!!!!

If you want to reuse the material a real good tool to have is a recprocating saw with a combination blade ie. metal/wood cutting. That way you can cutt the nails without splintering the wood. They are kind of expesive around $140.00 but worth every cent.