It is always advisable to obtain a mantra from a self-realized master. Until then we may use one of the mantras of our beloved deity like 'Om Namah Shivaya', 'Om Namo Bhagavate Vasudevaya', 'Om Namo Narayanaya', 'Hare Rama Hare Rama, Rama Rama Hare Hare, Hare Krishna Hare Krishna, Krishna Krishna Hare Hare', 'Om Shivashaktyaikya Rupinyai Namaha' or even the names of Christ, Allah or Buddha.