It is not possible to "clear" the mind completely.But you can clear it of all but one thought. The idea is to concentrate on one subject,or one object.If you are relaxed, sitting comfortably at a table. You then have one object on the table; nothing else. That is the object on which you will concentrate. It can be anything, from a lighted candle to a small statue.I was taught with a small bowl of water.Don't try too hard; just gaze at the object, and try to think only of that.
What happens then is that you will go into a sort of semi-trance [ your subconscious mind "takes over". All sorts of knowledge and ideas will come into your thoughts. You have to realise that your "conscious" sight sees only what you are looking at; it only "sees" a little of your surroundings,only hears what you "want" to hear. Your subconscious sees and hears everything! Everything you have ever experienced in your life is there "locked in". Meditation releases it!
You may have to practice a few times; but, eventually, you will "see" the magic!