How Cooking Works
You will need a fire of any type to cook food. To start cooking, click on a fire and choose the middle icon at the top of the screen.
Raw Meat
All raw meat should be cooked before eating because it 1) drastically lowers the chance of getting sick and 2) increases the amount of calories you acquire from most meats. Getting sick is always a risk (even with cooked meat) until your cooking skill is high enough.
Other Food Items
There are other food items you can "cook" over a fire, like Pork and Beans for example. However this is not necessary. It only warms up the food which gives you a boost to your warmth need meter for a while.
Drinks and Natural Medicine
If you have coffee, tea, or any natural medicinal ingredients in your inventory, you can brew them over the fire like you would a food item. You must have water in your inventory to do this. If you drink the brewed item right away, it will give you a warmth boost just like warming up a food item. If you don't drink it right away it will get cold and not provide the warmth bonus but will still provide the main effect of the item.