Violent sports

Banning violent sports is a highly debated topic in today's society. I think violent sports should be banned.
Firstly, violent sports such as boxing teach kids to use their fists instead of their words; children watch them on TV and see this as a pattern of behaviour. Secondly, people get seriously injured doing sports. Lastly, sport is an activity for pleasure and relaxing, not for hurting each other.
However, some people think violent sports shouldn't be banned. They say that a lot of people love violent sports and violent sport is a successful sports career.
But it's not like that. Violent sports are not as loved and attractive as they once were, for example compared to ten years ago, they now attract less and less revenue. There is no valid reason for not banning them.
Of course, there is no wrong or right opinion, but from my point of view, they should be banned

Beautifully written. I tried to correct the grammar only, while avoiding wholesale changes to text. - Hope this helps

Violent sports

Banning violent sports is a highly debated topic in today's society. I think violent sports should be banned.
Firstly, violent sports such as boxing teach kids to use their fists instead of their words. Children watch them on TV and see this as a patter*n* of behaviour. Secondly, people get seriously injured doing *these* sports. Lastly, *I believe* sport is an activity for pleasure and relaxing, not for hurting each other.
However, some people think violent sports shouldn't be banned. They *say* that a lot of people love violent sports and violent sport is a successful *sporting* career.
But it's not like that. Violent sports are not as loved and attractive as they were, for example, ten years ago.* Accordingly, they gain less and less money. There is no reason for not banning them.
Of course, there is no wrong or right opinion, but from my point of view, they should be banned.*

Violent sports

Banning violent sports is a highly debated topic in today's society. I think violent sports should be banned.
Firstly, violent sports, such as boxing, teach kids to use their fists instead of their words. Children watch them on TV and see this as a pattern of behaviour. Secondly, people get seriously injured doing playing sports. Lastly, sports is are an activity for pleasure and relaxing, not for hurting each other.
However, some people think violent sports shouldn't be banned. They said say that a lot of people love violent sports and violent sports is are a successful sport career.
But it's not like that. Violent sports are not as loved and attractive as they were, for example, ten years ago, accordingly, they gain produce less and less money. There is no reason for not banning them.
Of course, there is no wrong or right opinion, but from my point of view, they should be banned

Great entry overall! Your English was easily understood. 

People would usually say playing sports not doing sports, but either way it is easily understood. 

In the United States we would spell behavior as behavior not behaviour, but I will assume you are learning British English.