Paint
This is the fun part, paint! Before you get super nice paints, if this is your first time, I would recommend first buying some inexpensive paint to get accustomed to the process of working with it. In the section for Basic color theory, I will show you how to handle your paints.
Watercolor
Tempera
Acrylic
Acrylic colors dry as the water evaporates from it. Acrylic dries quick, it dries permanently (so if it gets on your clothes you will now have art clothes) and you will notices that it dries kind of plasticky (shiny). You can change their consistency with mediums and water. However, you do not want to add too much water to acrylic paints since you will be changing its chemistry. They do sell mediums (which are recommended) since its adding paint to more paint.
Depending on the quality of the acrylic paints, you will need more or less layers to make them opaque. More crafty paints require more layers, more professional paints require less since they are thicker and more opaque. Both are fun to use.
Crafts acrylic paint: dried quickly, needs several coats
Liquitex: heavy body acrylic paint