Concentration is purely a mental process. It needs an inward turning of the mind. It is not a muscular exercise. You should not fight and wrestle with the mind violently. When you concentrate on any object, avoid tension anywhere in the body or mind. Think gently of the object in a continuous manner. Do not allow the mind to wander away.
concentration can be done only if you are free from all distractions.
A man whose mind is filled with passion and all sorts of fantastic desires can hardly concentrate on any object even for a second. His mind will be jumping like an old monkey. Japa of any mantra and Pranayam will steady the mind, remove tossing and increase the power of concentration. Too much physical exertion…will cause distraction of mind. Those who practice concentration must abandon these things.
Whatever work you do, do with perfect concentration. Never leave the work without finishing it completely. You must train yourself to attend to the work on hand with perfect one-pointedness.… Reduction of wants and activities, renunciation of objects, solitude, silence, discipline of the senses, annihilation of lust and greed, control of anger, non-mixing with undesirable persons … all these pave a long way in increasing the power of concentration.
You should have real and intense thirst for God-realisation. Then all obstacles will be obviated. Concentration will be quite easy for you then.