This is the voxel editor, here you will find your operations: you may sculpt, paint or carve.
Notice that your hand UI also contains these options for easy access.
To operate simply reach your hand into the canvas and draw.
On the top panel, you will find your color wheel, palette, and brushes.
You can change what is displayed on your hand UI. To do this point at an element from the main menu and press Equip.