The first thing that you need is a good set of paints. It would help if you had an assortment of colors covering all the primary colors (red, yellow, and blue) and secondary colors (orange, green, and purple).
Next on your list are brushes or sponges. A good brush or sponge will make all the difference when painting, so invest in quality brushes/sponges if possible. Next are paper towels or rags to clean brushes/sponges.