What is to be meditated on?
Anything you prefer, but you should stick to one thing. Contemplation involves fight. As soon as you begin meditation other thoughts will crowd in, gather force and try to sink the single thought. The latter must gain strength by repeated practice. This battle always takes place in meditation.
Peace of mind is brought about by contemplation and through the absence of varying thoughts. Once dhyana is well established it cannot be given up, but will go on automatically even when you are engaged in work, play or even sleep. It must become so deep-rooted that it is natural.