You can earn easy money by selling fish. To do this go to the Market and select 'Shop' in the menu and then use the plus and minus signs to sell your stock of fish. It is also possible to sell Vegetables for Simoleons.
To cook in general you must first purchase a recipe from the local 'Corsican Bistro' which is located by the beach. When you have a recipe go to the grocery store 'Quickmart' to purchase your ingredients. If you have already upgraded your house you can grow ingredients in your garden which will save you time and money.