A practice
Mantra
Sound used as a practice — repeated until the repetition does the work.
मन्त्र
Mantra is the practice of repeated sound, spoken aloud, whispered, or held silently. What is repeated is chosen; the repetition is what changes the practitioner.
At this mandir mantra is heard constantly — during havan, during aarti, and in the personal practice of those who have been given one.
Beginning
Some mantras are public and anyone may take them up: the Ganapati mantra with which everything here begins, or the invocation of Datta Guru. Others are given personally by the Guru, and those are not repeated outside the practice.
If you want somewhere to start, start with ॐ गं गणपतये नमः, and start with a hundred and eight of them.