opengl game.need help in timer(or something)

hello.
i'm new to opengl.i'm making a game similar to tetris and i don't know how to make the pieces come down automatically.please help...

Tetris normally falls in increments:

Code:

void Game::Update(float timeDelta)
{
   m_elapsed += timeDelta;
   if (m_elapsed >= m_blockFallTime)
   {
       m_elapsed -= m_blockFallTime;

       // Tell current piece to fall down
       m_currentPiece.Fall()
   }
   ....
}

thanx