If it's not obvious, you can't actually access the hidden room from the start of the game. Once in a while. you might see a little ghost girl along a wall - that's the entrance to the hidden room.

In order to unlock it, you first need to spend souls on the "Disenchantment Bead" item at the soul shops.

Running along and checking every wall is a good way to do it but there's a better way that will save you time.

In the image below, you can see the secret room denoted by the "?" on the map.

Secret rooms have a few generation requirements:

1. It must not be adjacent to another room
2. It can't be in a "special" room (challenge, shop, etc)

With that said, you can see there are only FOUR potential spots the room could be in on the map below:

That's it. Good luck.