If you mean using food that you want to eat, go to "Inventory" and drag the food item over your character's mouth. Remember it must be cooked for it to have any real effect, if you want food cooked, go to the marketplace and select "Services - Cooking", this brings up a list of all chefs who are selling their services. Look at the food item and find out what level chef you need, then scroll down to that level category, services are listed from lowest to highest level and cost, the cheapest player will be first on the list for every level division. Then drag the food item over the chef's "Pot" icon and press Yes, remember you must have the money needed on you, it cannot be taken from your Storage.