Leashes are to keep you and your board within reach after wiping out. A leash should be about the same length as the board you are riding.

A light rub of board wax will help you keep a grip on your board and stop you slipping around too much. Particular attention should be given to areas where feet and hands are placed. Only wax the deck of the board.
When wax builds up on the board, a wax comb is used to freshen the wax surface and can be used instead of, or with a light rub of new wax.
Grip is used to supply better grip than wax (controversial) to the surface of the board and is best put on when the board is new. It can be just a tail patch or full deck kit.

Fins are either fixed, (permanent) or removable. One popular system of removable fins is FCS. Benefits of such a systems are that the fins cannot get broken off when travelling.
A soft plastic cap to put on the nose of the board to reduce the risk of injury to yourself and other people.
To protect the board while travelling. Two main types: a scratch bag and a travel bag. The travel bag is thick and protective against flight damage and baggage handling staff.