Interestingly enough, despite at first looking like a very important skill, Cooking actually is completely optional, as the characters don't have to eat in the game, and it gives no gains to actually eat food, cooked or not. It can be mostly seen as deepening the role-playing experience.
Therefore the skill can be ignored.
To train the Cooking skill, first learn up to 30 points at an NPC baker. After that, the easiest way to gather in this skill is to barbecue meat, which is the most simple meal that can be prepared. Be warned however, that even then to reach 100 points, around 20.000 barbecues are needed.