Hi Milaniyamiila and thanks for this essay.

A couple of small edits:

1. the government should pay due consideration on to enlightening people about leading healthy lifestyles
2. Top-level scientists in the field of medicine explain all repercussions causes by the repercussions of smoking, drinking spirits, overeating, eating junk food and so on.
3. testees -  it's correct but not common (and will make most 14-year-olds laugh, due to a similarity in sound to 'testes'). Perhaps you could use the word "participants" or "subjects"
4. Milaniyamiila  - do you have a clearer explanation for 'social advertisments'? I'm not really sure what you mean.

The more, the more:

Try to have the same grammatical structure (e.g. a noun, a noun phrase, a participle, etc) after each of the 'mores':

The more people are healthy the more money will be saved and can be spend on other aspects of life.

change to

The healthier the population, the more money that can be saved and spent on other aspects of life, such as education or housing. OR 

The greater the number of healthy people, the more money that can be saved and spent on other aspects of life, such as education or housing.

 Paragraph Organization

I would make a change in the organization of these sentences:

The other way of encouraging people to live healthy lives is newspapers and social advertisements.

It is proved by scientists that there is link between cancer and smoking. People who do not smoke less suffer from cancer. For example, recently there have been an experiment. Two hundred people addicted to smoking were offered to watch a social advertisement about smoking during two weeks.

I would omit the second and third sentences ("It is proved...", "People who do not smoke...") and join up the first single sentence with the paragraph: 

The other way of encouraging people to live healthy lives is through newspapers and social advertisements. For example, in a recent experiment, two hundred people addicted to smoking were offered asked/allowed/made to watch a social advertisement about smoking for two weeks...

Conclusion:

I would shorten your conclusion slightly:

In summary, population is the main aspect of existing of any state. Healthy population can be describes as a driving force of the economy. The more people are healthy the more money will be saved and can be spend on other aspects of life. Taking into consideration the above facts, the government certainly should allocate some money to educate the public about consequences of having unhealthy habits.

This could change to: 

In summary, a healthy population can be described as the driving force of an economy. The healthier the population, the more money that can be saved and spent on other aspects of life, such as education or housing. Taking this into consideration, the government certainly should allocate some money to educate the public about the consequences of having unhealthy habits.

Overall, a clear and well-written essay. 

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