By cooking, a player can make food for them while in battle. To train cooking, you have to fish for awhile. If you don't really want to train fishing, you might have to get around 300 lobsters. It is reccomended to cook manta rays, as they give the most xp.
Tinderboxes may be important to you if you want to get 99 cooking, since of course you have to cook food over a fire or range. You can buy a tinderbox at Skillz by the shopowner. If the shopkeeper isnt there because of some annoying person luring him, theres also a fire in WC.