If you are born a Hindu and grew up in India you worship a number of Gods and Goddesses. But the path of spirituality is through Japa/Mantra/Meditation. If you do Japa of many different Gods, it is not effective. Also the first step in spirituality is the realization that God is one. Who is this one God? Your Ishta Devata. Then you do Japa of that God with Mantra. As the mantras vary depending on the God, you choose the Mantra of the Ishta Devata. In meditation and Japa you have to visualize the form of the God. So you visualize the form of your Ishta Devata.
One God -- with form -- without form
Though many people may not even know the term Ishta Devata the process of progress in spirituality is the same. There are people who do not have an Ishta Devata and learn the mantras of many Gods. This fritters away your energy making it more difficult to reach a higher stage.
The whole process is about raising of the Kundalini Sakthi (Consciously or otherwise).
Choose a God/Goddess known to you. The process of visualization is also simpler.
We have a music season every year in Madras during which there are a number of Carnatic (south Indian Classical) music concerts. Some years back a Catholic Nun became our friend as she also attends these concerts regularly. Carnatic music is basically Bhakthi. When we were discussing the music the nun told us that her Ishta Devata is Lord Jesus and her Sadhana is through Carnatic music.
Hindus in India come to know about the different gods and Goddesses through the family and environment. But they also learn by visiting temples and through books/magazines. Books are a major source of information. The more gods you know about the more difficult the process of selection.
As per the classical texts the Guru selects the Ishta Devata if you do not have one. The process of selection also includes Astrology. You should be extremely fortunate to get a Guru like that.
I feel that I should have used the term "the ultimate principle" instead of God. Your Ishta Devata is the ultimate principle. Since it is easier to go from form to formless, you choose a form for the Ishta Devata.
We have misunderstandings when it is not realised that the ultimate principle is the Ishta Devata. When I say MAA KAALI is Brahman or Lord Krishna is Brahman it only means that the ultimate principle is in the form of MAA KAALI or Lord Krishna. Since I am comfortable with a form with compassion, love etc. I imagine MAA to be like that though I do realize that, since she is Brahman she is Nirguna. This is in conformity with my reasoning of going from Bhakthi Marga to Jnana Marga.
You concentrate on only one form at a time. To explain this I have to use the term Upasana. I am more comfortable with this term than with Sadhana.
Your Ishta Devata is your Upasana murthi. Though you may worship many Gods there should be only one Upasana murthi. I was warning about doing Upasana of many murthis at the same time. You may change your Upasana murthi depending on many factors. There is no harm in that. It does happen often.
Meditation requires concentration. You concentrate on the form of your Ishta Devata. Even for meditation on the formless many Gurus suggest imagining sky with a bird flying or open ocean.
Many people collect mantras like stamps. They also start Upasana of many Gods simultaneously. This leads to frittering of energy.