Re: a big, towering tree
The dandelion was blown to a city that has pollution troubles. It saw a little tree lamenting about her wretched lot, so he [You started the sentence with "It saw..", so you must carry on using "it". Or you could start the sentence with "He saw.."] asked her, “Why do you look so sad?” “I want to grow into a big, towering tree,”
Tthe little tree answered, and began to sob out her sad story. “However, I think it's going to be[or, "will be"] impossible to realize my dream because I am grown growing in this polluted, barren land.” “Believe in yourself. Never give up, and your dream will come true sooner or later,” The the dandelion said. “Really? Can I do it?” the little tree stopped crying and surprisingly asked in surprise. “Yes. I have confidence in you,” The the dandelion said. “Thank you. I will overcome,” the little tree said happily.
See this link for how to punctuate direct speech Punctuation in direct speech - Oxford Dictionaries Online
I'm not a teacher of English, but I have spoken it for (almost) all of my life....