How do I treat the floating-fly (black dots) in my sight vision?, I been having them in my left eye for 2 years now.

Floaters. They are most likely what are known as floaters. Floaters are condensations within the vitreous gel in the center of the eye. They can be annoying, but are usually not a significant problem. If you develop a shower of new floaters, particularly if seen with light flashes, that could indicate that you are developing a retinal tear or detachment, and you should be seen by an ophthalmologist asap.
Common problem. Many will observe some degree of floating objects in the visual space which represent membrane fragments and opacities in the vitreous cavity in the back of the eye. These are usually benign, especially at your age, and there is no treatment. You will learn to ignore and compensate for yours.