Here are the most essential brushes to get you started with Paint It Simply Techniques. 

Often in these lessons, the shape and the exact size of the brush is interchangable. You can substitute a flat for a filbert or a #4 with a #6 and still get a great result. However, the most important aspect of these brushes is the hair that they are made from. Constructed from a synthetic squirrel hair, they are very soft and hold a lot of moisture. They are very responsive to the pressure from your hand and will paint a vast array of subjects and surfaces. Wash with soap and water. Dress with a touch of Extender Medium, if desired, for added protection against drying paint.

This set includes:

GAB13- 3/4" Watercolor Flat

GAB38- #6 Fusion Flat

GAB01- #4 Watercolor Round