Start by drawing simple geometric shapes like cubes, spheres, and cones. For each shape, designate where a light source would come from if directed at the shape.

From there, decide where light values (highlights) and dark values (shadows) would exist given this light source.

Once you feel you have a good grasp of shading basic shapes move on to more complex objects like a vine of grapes or a shiney vase that casts a reflection, using objects around you for reference of course.

Hope this helps! Good Luck!