if you don't have something/anything nice to say, then don't say anything at all!

I agree with the sentiment expressed in this statement. There are some situations that where/in which your criticism can lead to/result in very bad unfortunate consequences. Sometimes It could damage the confidence of/do psychological damage to destroy phychology of teenagers. You must think before you say something anything that is potentially hurtful. It (what is 'it'?) Communication is an amazing skill for communication. or: Forethought/Thoughtfulness is an important communication skill? You should ponder every next step in your conversation.
Sometimes people just don't want to know your opinion, so you must keep your criticism to yourself. We have to be more patient and polite to each other and only then we will have and keep many good friends.
Every person has their own style and favourite clothes. Someone likes to wear jeans or T-shirts and another one prefers dresses or romantic(?) style/fashion. So you shouldn't say people bad things about their people's appearance because it is so harsh and thoughtless roughly and cruelly (adjectives, not adverbs!).
Thus we will try to be more kind and merciful to others.

"If you can't say something nice, don't say nothing at all". Fantasia, as spoken by Thumper.

http://en.wikipedia.org/wiki/Thumper_(Bambi)

if If you don't have something nice to say, then don't say anything at all!

I agree with this statement. There are some situations that where your criticism can lead to very bad consequences. Sometimes it can destroy the phychology of teenagers. You must think before you say something. It is an amazing skill for effective communication. You should ponder every next consider each step in of your conversation.
Sometimes people just don't want to know your opinion, so you must should keep your criticism to yourself. We have to be more petient patient and polite to each other and only then we will have many good friends.
Every person has own style and favourite clothes. Someone Some likes like to wear jeans or T-shirts and while another one others prefers prefer dresses or romantic style. So you shouldn't say people bad things to people about their appearance because it is so roughly and cruelly. cruel.
Thus we will try to be more kind and mersiful merciful to others.