What is the most effective/easiest way to learn how to draw professionally?
By definition, no art pursuit is easy. If art were easy, then it would not be important in our society. Think about it this way: no one thinks that tying one's shoelaces is anything special because most people can do it pretty easily. We enjoy the arts precisely because we know from experience how challenging art can be. We enjoy professional sports for the same reason. It's a joy to watch a talented athlete or look at Da Vinci's drawings.
There is a way in which drawing can feel easy some of the time, however, and that is to have a complete passion for doing it. People who have a passion for their art are much more willing to sit down and do the practicing it takes in order to master that art. They enjoy the process of doing something and then analyzing it to see what they could do better the next time. They love learning about techniques and then drawing and drawing and drawing to perfect those techniques. They spend many of their waking hours thinking about art.
If you feel this way about drawing, take some classes with increasingly talented teachers. Read books about drawing, go to museums to see original art works, peruse the art you cannot get to via books and internet, and practice. Be in it for the long run, and you will definitely improve.
Drawing isn't a talent picked up easily. It requires many hours of practise to draw like a professional.
Having the ability to translate and interpret what you see in the drawing is a critical skill. Having the ability to correctly draw tone, line and shape is by far the most difficult to learn, yet most essential key to drawing like a professional. A good way to acquire this skill is to practise lots. It may seem like the obvious way to acquire this skill but it is definitely the most practical.
Materials are also an important factor of how well your drawings quality may be. Find the right pencil for you. You may like a firm pencil or a softer pencil. Your hand will be more relaxed if you have the correct pencil meaning that your mind will be more concentrated, thus giving you a better final result. Paper wise I suggest using smooth bristol. One side is smooth and the other side gives you texture, and there are so many shapes, sizes and colours.
The subject is also a fairly important thing that you should consider when aiming to get a high drawing quality. If you are drawing something you are passionate about it is more likely you will get a higher result. For example I like the look of crotchet notes in music, if I draw these I will get a higher result, but if I drew something I disliked, for example cartoons, I would get a lower result.
There are many other things to think about when learning to draw. A few important ones I suggest thinking about:
Have fun drawing and I hope if you are learning to draw that you get the best result possible. Good luck!