It depends actually. It depends on whether that something is true or not. If it's true and many people believe it, then it is. If it's fake but many people believe it, it's not true.

Of course, things are not that simple.

If you are the unknowing one...
Let's say that person A does not like person B and makes up some strange, untrue story to defame her. Person A happens to be a popular person in the student body of her school. So a lot of people believe her when she says that person B did this or that.
Then, there are the cynics. They start to ponder over the truth in person A's story. Perhaps some of them might be popular too. So they start dispelling the possible lies, explaining what is and not.
This would result in the confusion and indecision of the rest of the student body who have really actually no idea who to believe...

So is it true or not? Just because many people believe that something is true...
Well, no, obviously.