1. It's about Tom's worries on his height and changes in teenagers' body and mind.
2. It's about Tom's worries in his height and changes in teenagers' body and mind.
(Which sentence is right? Do I have to use 'on' or 'in'? Thank you.)
3. He is 170 cm in height.
4. He is 170 cm high.
5. He is 170 cm tall.
(Are all the same?)
6. I am going to New York tomorrow.
7. I will go to New York tomorrow.
8. I go to New York tomorrow.
(Are the three the same?)
9. Sometimes, you want to be alone.
10. You sometimes want to be alone.
11. You want to be alone sometimes.
(They are all grammatical, aren't they? By the way, do we have to put a comma after 'Sometimes'?)
For 1 and 2, I would use "about." Is Tom concerned about all teenagers or just himself? If just himself, I would use "a teenager's".
I would not use 4, but 3 and 5 are equivalent.
8 is not good, but 6 and 7 indicate future actions.
9-11 are grammatical and the comma is not needed.
However, most people use "you" when a more accurate statement would use "I". It is a way of not acknowledging the one's own feelings or desires. We are not sure of others' feelings or desires, but we are likely to be sure of our own. Using "you" attempts to give the assurance that others share your feelings. Using "I" takes ownership of these feelings or desires. The use of "you" reduces the chances of being judged for what is felt or desired.
I hope this helps. Thanks for asking.