An American English perspective, to complement the British English above :)

Dear Sir,
I knew heard about your plan to open a fast food chain will open in my town. I’ll try to convince you not to do it.
5 reasons why you shouldn’t open a fast food chain in this town:
1. There are already a lot of fast food restaurants in this town. It isn’t won't be as popular as it can could be elsewhere.
2. Because of unpopularity in this place here you will lost lose a lot of moneys money.
3. More and more people are against of fast food now. Would you like to see a negative article in a magazines? (Depending on your intended meaning, either) Haven't you seen the negative articles in magazines? (or) Would you like to see negative articles, written about you in magazines? (or) Would you like to see a negative article, about your restaraunt, in a magazine? (I'm not clear what you meant exactly).
4. Making people fat is not cool.
5. If you open a sport centre center (American spelling), you will do make people happier and healthier. Here allowed to be You could have lots of sports equipment. and it will be if you make it cheap for people., This people they will talk about this place and you’ll become the most famous person in a town.
The right choice depends on is up to you.

Best wishes,
Kate.

Please, Find mistakes here. It's for school.

Dear Sir,
I knew heard about your a fast food chain that will open in my town. I’ll try to convince you not to do it.
5 reasons why you shouldn’t open a fast food chain in this town:
1. There are a lot of fast food restaurants in this town. It isn’t They aren't as very popular as can be.
2. Because of this unpopularity in this place you will loste a lot of moneys.
3. More and more people are against of fast food now. Would you like to see a negative article in a magazines?
4. Making people fat is not cool.
5. If you open a sport centre, you will do make people happier and healthier. Here allowed to There would be lots of sports equipment and it willould be cheap for people. Thise people will talk about this place and you’ll become the most famous person in a the town.
The right choice depends on you.

Best wishes,
Kate.

Kate, you certainly have the right idea! Thanks for writing this! I wish young people in my country were as smart about fast food as you are! I am glad to help you with your message.

Please, Find mistakes here. It's for school.

Dear Sir,
I knew about your a fast food chain will open in my town.I understand that you are thinking about opening one of your fast food chain restaurants in my town. I’ll I am going to/I want to try to convince you not to do it/this.
5 reasons why you shouldn’t open a fast food chain in this town:
1. There are a lot of fast food restaurants in this town. It isn’t They aren't as very popular here.as can be.
2. Because of Due to the fact that fast food restaurants / Due to the fact that they are unpopular in this place town/area you will lost lose a lot of moneys. When we are talking about money it is always used in the singular in this type of context.
3. More and more people are against of fast food now. Would you like to see a negative article in a magazines about your restauarant chain?  If you are meaning to ask if the reader reads negative articles about fast food chains in general then - Have you read/Have you seen all the negative magazine articles about fast food? 
4. Making people fat is not cool.      
5. If you open a sport centre, you will do make people happier and healthier. Here allowed to be  you could have lots of sports equipment and it will be make it cheap for people. This people They will talk about this your place and you’ll become the most famous/respected/revered person in a town.
The right choice depends on you.It is up to you to make the right choice. 

Best wishes,
Kate.

Very good points to make a case against more fast food restaurants opening

Please, Find mistakes here. It's for school.

CORRECTION:    

COMMENTS:    

 * I crossed out what you wrote and  put my corrections in.

Dear Sir,
I knew have heard about your a fast food chain which will be opening in my town. I’ll try to convince you not to do it.
5 reasons why you shouldn’t open a fast food chain in this town:
1. There are a lot of fast food restaurants in this town. So your one isn’t won't be as popular as it could be somewhere else.
2. Because of unpopularity in this place you will lost a lot of moneys.
3. More and more people are against of fast food now. Would you like to see a negative article in a magazines?
4. Making people fat is not cool.
5. If you open a sports centre, you will do make people happier and healthier. Here allowed to be lots of sports equipment Sorry, I don't know what you mean by that. You can put lots of sports equipment in it and also, it will be cheap for people. This Furthermore, if you do this, you people will will make lots of people talk about this place the centre and then you’ll become the most famous person in a town.
The right choice depends on you It's more natural to say, 'It's your choice.' 

Best wishes,
Kate.