Being religious might help one to achieve growth externally, being spiritual goes beyond that to achieve inner growth. Mantra Japa (Repetition of a Mantra) is one such spiritual practice that achieves inner growth in addition to external results. Japa is also equated to a sacrifice one does with one's own prana (vital energy) through chanting. In Bhagavad Gita, Krshna exemplifies the importance of connecting to Ishvara (God) through chanting by equating himself to Mantra Japa. According the scriptures, Mantra can be defined as the process of using the power of one's own mind to protect oneself. Mind creates our world we live in and a strong mind creates a better world. Hence mastery of one's mind in such a way that it protects us is the focus of mantra chanting.
One may choose a mantra which connects with her the most. This could be based on a favourite deity (Ishta Devata). This creates an emotional connection (Bhava) with chanting. Choosing different mantras to chant during different stages in one's life is common as one's tastes, challenges and goals change along the way. Horoscope (jataka) based selection does not provide this flexibility of selection. If you have taken Mantra Diksha (You have accepted a Guru based on clear understanding of her and have been initiated into a specific mantra), you will continue to follow her instructions in chanting the mantra.
What is needed for chanting is a Japa Mala (Beads used for chanting), kept inside a pouch specifically made for this purpose. Mala comes in many shapes and forms - Rudraksha, Sphatika (crystal), Mani (gems), Chandana (sandal), Tulsi are commonly used ones. Japa mala selection is based on personal preferences, ask for advice if needed. For a beginner who is not sure of which one to use, Chandana Mala seems most appropriate. Make sure you have 54 or 108 beads in your mala (there is one additional bead for a japa mala) and the beads are not damaged. Japa Mala is not exchanged usually with another person. It can be kept inside the pouch or worn on your body if comfortable. During chanting, one sits on a seat, and not on the floor.
One may remember Ishvara in many situations - when faced with distress, in need of wealth/health/prosperity or when in need of knowledge. Our scriptures describe the various goals of a seeker and clarify that there is no gradation between them. One may seek Ishvara, based on one's needs and goals. Chanting a mantra as mentioned before is an activity of repeatedly remembering Ishvara, results in inner purification (mind) and achieving external results. It is very important for the individual to be clear on what one wants to achieve through chanting. One needs to do a bit of homework to figure this out - the clearer the goals, the better the results will be. More over, the goals dictate what and how chanting should be performed. - The Hindu Hub