The early painters used wood panels as supports for their paintings. Canvas came along later. I like painting on Masonite panels treated on both sides with gesso. You can also use primed canvas if that is your preference.
Gesso is a quick drying acrylic paint that is used to prime what ever surface you choose to paint on. As it drys it shrinks, so by priming both sides of a rigid surface like Masonite it prevents warping. I apply a minimum of three coats alternating the direction of brush strokes by 90 degrees between each coat.
After the gesso has fully dried I sand the surface with fine sand paper to get a smooth surface. While some sanding is necessary how smooth you sand it is a matter of personal preference.