To create this look, take a few pieces of old wood and nail them together. If you have to buy new wood, sand your wood roughly to even the surface. This will prevent your clothes and other material from tearing when leaning against the board.
Paint the wood with a few brushes of paint here and there to give it some character and to give it that 'shabby' look. Bare in mind that the more distressed, the better for a look like this. Once you have your panel of wooden slats, paint your message of choice on and set it against your bed.