Англичане проверьте текст на ошибки?

Ошибок достаточно. Многие -- грубые. Ну и литературная ценность данного произведения не оценивалась. А так, вот, катай:

Smoking is one of the most important issues of 21st century.
In the last few years smoking has been going up among teenagers.
The most worrying trend is an increase in quantity of cigarettes.
One of the possible reasons for this could be the fact that a life of people became more difficult.
Another reason is that teenagers wish to look like adults.

Smoking is a process that is physiologically necessary for a human body. Under the influence of nicotine the old and weakened brain cells die. Without smoking this process can be received only by intensive engaging in sports or difficult respiratory exercises.
Process of smoke inhaling and cough of the smoker is respiratory gymnastics.
The smoking person is better adapted for a life. In a critical minute there will be always matches in his pocket to light a fire.
The smoker does not have problems if he would like to start conversation or get acquainted with someone.
The only thing he need is to ask a spark.

The impacts of smoking on human health are serious and in many cases deadly.
There are approximately 4000 chemicals in cigarettes, hundreds of which are toxic.
The effects of cigarette smoking are destructive and widespread.
Toxic ingredients in cigarette smoke travel throughout the body, causing damage in several different ways.
Smoking affects the function of the immune system and may increase the risk of respiratory and other infections.
The body produces antioxidants to help repair damaged cells. Smokers have lower levels of antioxidants in their blood than nonsmokers have.
Smoking is connected with higher levels of chronic inflammation, another damaging process that may result in oxidative stress.

In my opinion tobacco is a terrible drug. It is better to remain the healthy non-smoking person.
Based on all things mentioned I believe that smoking kills people.

"Another reason is that are teenagers wish to seem adults."
Another reason is that teenagers wish to seem adults
"are" не нужно сдесь

"Smoking - process physiologically necessary for a human body."
Smoking - process is physiologically necessary for a human body.
глагол где?) ) вот сдесь is как раз и надо)

"Without smoking this process can be received only intensive playing sports or difficult respiratory exercises."
Without smoking this process can be received only by intensive playing sports or difficult respiratory exercises.
забыли by

"A critical minute in its pocket always there will be matches to dissolve a fire."
я бы написала
There will be always matches in its pocket to dissolve a fire in a critical minute.

"The smoker does not have problems if he wishes to start talking, get acquainted with someone."
может лутше wants вместо wishes

"The effects of smoking on human health are serious and in many cases, deadly."
effects не очень мне нравится, сonsequences

что нашла поправила

You done a good job and I will pass you anyday and to fix it will only change the meaning