Which one of them is true? Please help me. Thanks!
It depends usually on whether you are specifying the thing that you found out, or using "it/that" etc.
Yesterday, I
found out that I had passed my exam.
I wanted to
find out what mark I had got in my exam. Yesterday, I
found it out.
How did you
find out that there had been an earthquake?
I
found that out by reading the newspaper.