This is a very frequent question. The tradition in India is that you either have a mantra guru or a spiritual master who will give you your personal secret mantra, or if you don't have a guru, then what it's done is to start practicing different mantras, and see how you resonate with them. It is advisable to practice them for at least 21 or 40 days. If after this period, you start feeling connected with the energy of the mantra - let's say the mantra Om Gam Ganapataye Namaha for Ganesha, then you can choose it and continue with it every day.