Many artists prefer the idea of pencil sketching either instead of painting or in addition to other art forms. You may find some people who prefer to create pencil sketches in order to prepare themselves for whatever art piece they are going to paint or color. In reality it is much easier to create a three-dimensional drawing with pencil sketching than it is to attempt to paint it on a canvas or other material. By sketching your idea first, you can then fill in the details and thus make your creation appear more real. You will use a method similar to what you learn in but instead of using the two-dimensional flat shapes you want to find the three-dimensional ones.
Do not attempt complicated objects when you first begin to draw. Start with simple objects and then work your way up to more complicated ones. You may want to try imagining what you want to sketch is made of glass and then develop a visual image of the object's hidden edges in order to be able to draw them. Before you begin your actual drawing you want to lightly pencil in your entire form in order to help you see the object's perspective and proportions.
There are two different ways you can approach the process of drawing structure. One way is to begin with a basic skeleton and then add detail as you visualize the basic shapes that lie underneath a more complex surface much like a sculptor who adds pieces to a clay structure. The other approach is to work from the outside in allowing yourself to create an imaginary image of the basic shapes that are necessary to fit within the structure. Most artists will use a combination of these two structures rather than attempting to use one or the other.
When you are beginning pencil sketching whether as a sketch artist or as a first step to something different, make sure you begin with something simple and work your way up to something more complicated. It's easier to learn the basics of something simple and then be able to add in more complex details as you become more experienced than to attempt to begin with something complex and then become frustrated because you are unable to complete the project you began.
During the learning process you want to choose some simple shapes and objects and then build off them until you are able to build them into something more complex. This is especially important if you are attempting to build a three-dimensional figure, something more complex than a one or two-dimensional figure. Certainly it is more lifelike, but it also takes more time and experience to complete. You don't want to rush through your drawings but rather take your time so that you can develop a feel for the images as you continue. The more experience you gain the easier it will be for you to draw the more complex objects and images and even develop your drawings into something more profound and even colorful.