Do i have to prime my ceiling if it had stain on it I'm going white

Here is lots of information on your question. Happy painting!

https://www.youtube.com/results?search_query=how+to+paint+white+over+stain

You can prime the stain only or you can prime the whole ceiling depending on how many stains you have and how bad they are. Sometimes just the paint--preferably two coats--will cover the stains but if the stains are really dark I would prime first.

Unfortunately, I think so. Depends weather they were sealed or not as to what you should prime with. Ask what product to use at the paint store.

I would definitely recommend that you prime the ceiling first before putting your finish coat on. Try Zinsser Bullseye 1-2-3 primer.

Definitely yes. I would recommend a product like KILZ to block any stains from bleeding through.

I agree. I always prime when I paint anything. Always a possibility of bleed through. Also makes the final finish more vivid and true.