Notice very carefully the following (you are looking at the simple glass of water on the table):
Size (in relationship to something, perhaps the table it's on)
Shape (this is KEY --vital-- to drawing skill) Is it a circle, oval, cube, tube, pyramid, cone or some combination of these or other shapes?
The glass will probably resemble an oval at the top with a cylinder shape down to the bottom of it. Where the water is will show a faint oval outline at the "waterline" (top of the water).
Even if you're drawing this in black and white (start with that first), NOTICE:
Color (or transparent),Texture: Soft, rough, smooth, shaggy, etc., shadows or light falling on it. Now for the proverbial question --- The glass: Is it empty or full? In between? ("Is the glass half-empty or half-full?")