There are different methods in Vedic Astrology to know the ‘Ishta Dev’ or main Deity. But in my own experience I am selecting Moon to decide the Deity.
Moon is our mind , our inclination, our liking disliking, our mood, our taste, our natural attraction. Moon is the only planet who represents our Astral body for our so many past births.
So check your Moon in your birth chart and see his connection with other planet. First by conjunction, if no planet with Moon then see opposition, if no opposition then see if any planet is aspecting Moon or Moon is exchanging sign with any planet and if that also not there then check Navamsha chart to see Moon connection.
If Moon is with Sun then your soul has much progressed . Meditation will be your main liking. You trust the God who is formless. If Sun Moon conjunction in 6 8 or 12 house then it is weak. In this case Sun or Gayatri mantra is best.
Mars is the energy, the Shakti. When Moon is connected with Mars you love to any form of Goddess. You love Jagadamba. Any Shakti upasana like Amba, Durga, Kali, Chandika or Kuldevi is best.
Mercury is the lord of dual signs so if Moon is in odd sign and connected with Mercury then Vishnu, Ram, Krishna like male Deity and if even sign then Saraswati upasana is best.
Jupiter is also the lord of dual signs so Moon if in odd sign then take initiation or Dixa from GURU and worship Guru or dixa mantra or Shiva upasana. If in even sign then Matru upasana or any form of Jagadamba. (Moon Jupiter conjunction or exchange loves mother very much. Sun Jupiter conjunction loves father very much. )
Moon Venus in odd sign Krishna mantra, Swaminarayan mantra, Shri Nathji bawa worship is best. If in even sign then Laxmi or any form of Jagadamba worship is best.
Saturn is the last planet who is dwelling in Sahasrar Chakra and takes the soul to Nirvana so Shiva is the best selection for him. ‘Om Namah Shivay’ is best for Moon Saturn conjunction and opposition.
In every temple we start from left Ganesh ji (Ketu) for blessings and end with right side Hanuman ji (Rahu) for devotion or bhakti . Ketu with Moon or in Lagna suggests Ganesh ji Worship whereas Rahu with Moon or Lagna suggests Hanuman ji worship.
Weak Moon if with Saturn or Rahu in 6 8 or 12 then it creates fears. They people do not recognise the right Guru and they are attracted to Bhuva, Tantrik and so called hypocrite and imposter baba ji and worship lower level Deities.
If Moon is in Fiery sign then Homan or Sacred Fire, If Earthy sign then donate to Cows Dogs and Poor people, If Airy sign then Mantra Japam and if watery sign then Abhishekam of water on the deity .
I think this much details is enough to understand One’s Deity and worship.