After you get the outer shape of the object (or perhaps the major inner lines and shapes too), then concentrate on adding the following:
Which side of the glass is darker (in shade or away from the light source)? You can suggest this shading and roundness of the glass with soft, smudgy lines or what we call "cross-hatching" (fine lines which form a shadow). Don't overdo the shading until you have more experience, as it can get "dark-looking" quite fast. Subtle and soft shading works best when you are just starting out.
Fill in some of the differences in the clear water inside the glass, softly giving it a bit of shade every so often -- don't at this point in your Drawing Development put in imaginary lines and shadows -- there will be plenty of time for creative imagination once you learn to draw what's there in front of you as accurately as possible. You will develop an awareness of "design" (where things fit) this way.
