How to cookFollow

Flint & tinder are best when you go someplace whre there isn't already a fire. Most towns (not all, unfortunately) have places where you can cook - it might be a fireplace, a big bonfire, a cast-iron stove, a brazier, or something similar. If you have to resort to flint & tinder, open your spellbook and click on "Basic campfire" <--If I recall correctly

Yes, as Vole said you also need wood to make a fire, and as Mike says, you can almost always find a fire. Even in Astranaar, which I thought was really odd for not having a fire anywhere, if you go right across the lake there are some abandoned camps with campfires that work just fine. Most of the inns (except for the night elves) have a fire, most houses for humans and I think all of the dwarvish ones have them, and Ironforge has one every few feet. The cooking trainers are all standing next to some form of fire.

To start a fire, look in your spellbook for basic campfire, to cook, look in your spellbook for cooking. It will show all the recipes you currently have, and tell you what ingredients you need for each dish.