Short answer
Enter the baby's date, exact time and place of birth in the Moon-sign finder to find their Nakshatra (birth star) and pada (quarter). Each of the 27 Nakshatras has four padas, and tradition gives each pada its own starting syllable, so the pada decides the letter. Check which naming list your family follows, as regional lists differ.
How ChiChakra reads this
The Nakshatra comes from the Moon's position at birth. The zodiac is split into 27 Nakshatras of 13°20′ each, and each into four padas of 3°20′. The Moon crosses a pada in roughly six hours, so the recorded birth time matters.
Choosing the name
Take the time from the hospital record rather than memory, and use the town of birth.
Look up the syllables for that pada in the list your family uses. Tamil, Telugu and North Indian traditions spell some sounds differently, and none of them is wrong.
Many families choose a formal name from the syllable and a different everyday name. Both are fine.
If you're also planning the naming ceremony (Namakarana), choosing its date has its own traditional checks.
Good to know
A birth star guides tradition; it isn't a prediction of your child's character or future.