I want to do mutah, with conditions, with someone, to get to know him.
My dad and grand father passed away.

I heard that I dont need permission from anybody. It still be a halal relation.
But I also heard that I need permission from my guardian.

What is the fiqh law please?

One should refrain from Mutaa and get involved in a permanent marriage and it is strongly adviced in such as your case to not let the guard down because there are many out there they will misuse the woman and abuse her in the name of the Muta. So please be careful.