1. Is it correct to say you was or you were?
you were
2. Is it correct to say he/she/it will has or he/she/it will have?
he/she/it will have
3. He was in Paris last year.
I was in Rome last year. -is it correct?
Yes, that is correct.
4. I have been in New York. I had been in New York. Is it correct and what does it mean? In which cases is it possible to use?
Use "to", not "in". "I have been to New York." and "I had been to New York." are both grammatically correct, but have different meanings.
Use "have been" to talk about most things in the past. Use "had been" when you are telling a story about something that happened in the past and need to talk about something further in the past before your story.
Look up "english present perfect vs past perfect" for a better explanation.
5. I been in New York. is it possible to use?
No. "been" is a past participle, so it must always be used with a form of "have".