Q: Am I to keep on repeating "Who am I?" so as to make a mantra of it?
A: No. "Who am I?" is not a mantra. It means that you must find out where in you arises the "I"-thought which is the source of all other thoughts.
Q: Shall I meditate on "I am Brahman" (aham Brahmasmi)?
A: The text is not meant for thinking "I am Brahman". Aham ("I") is known to every one. Brahman abides as aham in every one. Find out the "I". The "I" is already Brahman. You need not think so Simply find out the "I".

aus
Be as you are
The teachings of Sri Ramana Maharshi
edited by David Godman
a Penguin Book