Q: Can you please clarify as to who is the 'I' and who is the real meditator, 'I' or the mind?
A: 'I' (as pure self) do not meditate as 'I' is always in a changeless, non-modifying, attributeless and thoughtless state. Thoughts do not emanate from 'I' but they emanate from mind. It is the mind who attempts meditation so as to become calm. Do not get carried away by the gimmicks, which the mind resorts to, just because it is unable to be stable. Try consciously to be always aware of your 'I' (the real self) and the mind will start taking rest in you."Meditation is not meant for the meditator (the Self, Atman) and meditator is not at all meditating but meditation is meant for the 'Mediator', the mind, whose phenomena is to identify with everything other than its own source and thus veil the conscious existence of an entity called the self, seer, experiencer, meditator, atman, brahman and so on..."
Q:How do we know which mantra to chant? Should the mantra be chanted loudly or mentally?
A: There are several common mantras which anybody can chant such as OM, OM NAMAH SHIVAY, OM VASUDEVAYA NAMAH, OM GANESHAYA NAMAH, OM HAMSA SOHAM etc. You can also take a mantra from your Guru or refer scriptures. You have to be careful only when you need a mantra for fulfilling any specific desires of material nature.
Mantra can be chanted loudly if you are in a place where others do not mind or it does not unnecessarily attract others attention. Mental chanting of mantra has more power than chanting it loudly through mouth. While chanting mantra you must feel its divine meaning and concentrate on its vibratory effect. Every mantra which is chanted should give you joy and take you to a state of meditation. It should be slow and rhythmic chanting. The more you chant mantra the more powerful it becomes
hence keep chanting it whenever and wherever it is possible. You need not restrict mantra chanting to a particular time and place.