The basic drawing tool in modern times is the pencil.
To be able to draw and shade in a professional way, it is necessary to be able to sharpen your pencils correctly.
Sketching is an important part of every creative day. The better you can manage your tools the better your end-product will be. A dull pencil will set you up for frustration, not to mention the quality of your work.

For beginners I do not recommend buying the most expensive pencils to start off with.

Start with a decent artist quality pencil.

Any decent pencil should have a lead that does not break too easily and that can be sharpened to a really pointy tip. As these pencils are quite an investment you would want to get the most out of them. Using a sharpener, you will loose a lot of pencil. The best way forward is then to learn how to sharpen your pencils with a blade or knife.

To be able to use the pencil for shading you would want to cut away more wood from the graphite, and so using the exposed graphite.

To sharpen your  graphite pencils:
You need a pencil, traditional sharpener, thin blade or utility knife, fine grit sand paper.

If the pencil has not been sharpened before, use the traditional sharpener to start of the process.
Hold the blade in your strong hand and the pencil in the other.
Use long cuts and push the blade in long motions to shave the wood. Do not cut to deep. If you press too hard the graphite might break.
Thin shavings will take longer but in the end you will have a sharper pencil. Rotate the pencil in your hand as you shave the wood.
I tend to shave the one side first and then turn the pencil 180degrees to do the other side.
Keep on shaving little bits until you have evenly removed the wood.

Charcoal pencils can be shaved a bit longer. Try to get the exposed charcoal no more than 1.5cm (just over half an inch). This will give you the most control over your drawing tool once you are busy sketching. If it is too long it will easily break while you draw.
Graphite pencils are generally stronger and you can easily expose more of the graphite (2 cm, or ¾ of an inch)
Once you have done that you can use a sandpaper block (or just fine grit sandpaper) to sharpen the exposed graphite. Just rube the graphite on the sandpaper, while at the same time rotating the pencil. For detailed work you can use the sandpaper continuously to sharpen the tip of the pencil.

By sharpening you pencil this way you have a needle-point for detail work, and also the sides of the long exposed graphite head to do shading.
The final part of sharpening can also be done with a blade, if you do not have sandpaper handy. Slowly scrape the blade bled on the exposed graphite whilst turning the pencil in your hand.

As you use the pencils, try and turn them in your hand, thus wearing the tip off evenly.

Inevitably you will break some pencils in the beginning. Keep practicing.