How Does A Negative Person Become More Positive?

A negative person won't become positive unless they want to work at it.  Sometimes people are constantly negative or miserable because that's all they know.  They find comfort in what's familiar to them, and that includes negativity.  

If you work with young people, you might try to help them feel better about themselves.  Negativity is often the result of poor self esteem.  They have to know that they are accepted and have worth as human beings.  

It's difficult to be more specific.  You didn't indicate the age group you work with or whether or not they have any kinds of problems, but I wish you luck.