I have an assignment and i need someone to correct my writing

This story is for kids , so don't be surprised :)

[ The Body Changeling ]

One day, there was a fat boy called John playing in the garden with his ball. He was very kind boy but his only problem was eating too much fast food and candy (when you are talking about candy in general you would say candy. When you are talking about multiple specific candies you would use candies). This problem was the reason why his friends underestimated him all the time. (I changed underestimate to underestimated because you were using past tense throughout this paragraph and this sentence and you need to match verb tenses.)

While John was playing, his friends saw him and started laughing at his corpulence. One of them, called Tom, who the closest friend of John, was always sad for his situation so he decided to talk to him. Tom said, "Hey John, what's your problem dude? How long do you want to stay this way?" John said, with a sad expression, "Huh! I don't know!" Tom replied, with an angry expression, "I will let you go now, but think about it and you will not regret it." Then he went away. (I changed the third said to 'replied' to make the paragraph more interesting. When you write "said" all the time it gets repetitive, so you should use synonyms to make things more interesting. I changed "anger expression" to "angry expression" but you could also change that to "He replied, angrily,")
After that, John started thinking. He stared (changed stares to stared to match verb tenses) at the sky and said, "He's right!" Thus he decided to work out (when you are talking about the verb, you would write 'work out'. When you are talking about the noun, it is workout. Example: "My workout yesterday went really well, so I am going to work out again today!") daily, follow a hard strict (I changed strict to hard because hard in this sentence would mean "difficult." Strict diet is a common phrase in English and I think might work better in this sentence) diet, and get away from his friends for a period of time. (This is not wrong but it's more common to say "get away from his friends for a while.")

After 6 months, John came back with a very nice body. He went to the same garden which he played in it (you should not use "it" here; it doesn't make sense) and saw his friends again. They looked (I changed look to looked to match verb tenses) at him but they didn't know who he was (changed is to was to match verb tenses)! but (you do not need "but" here) When he started talking to them, they knew him immediately and were really shocked. Then they gathered around him and congratulated him and forgot the old John whom knows (I'm not sure what you mean here by "forgot the old John whom knows"). Then his close friend Tom finally said, with a big (you do not need big because huge already means big) huge smile on his face, "Great job bro!"


( Sorry for the long story )

I edited the story's punctuation, especially surrounding things people said, to match the way stories are normally written. There were some extra spaces around punctuation marks that I got rid of as they are not needed. I separated sentences and created new ones out of them. It is not good to combine sentences and separate them with commas; it is better to start new sentences completely. 

I hope that helped!