When your health is worn down from fighting, cooking is the main skill that provides food you can consume to bring yourself back to full health.
To cook standard fish, you need to find a campfire, available in most towns.
First, click on the raw fish you want to cook to equip it (Box edges turn yellow)
then click on the FIRE. Successful cooking looks like this:
Each cooking level above the required base level gives an extra 1% success rate. For example, at level 1 cooking you have 50% to cook raw perch; at level 20 it would be 70%.
The kettle allows you to make mixed foods, and is also available in many towns. All raw ingredients have to be in your inventory.
In the list below, all names that are blue can be cooked on a Campfire, and all names that are orange can be made on a Kettle.