Your birth star, pada & ruling planet.
Nakshatras are the 27 lunar mansions of Vedic astrology, each 13°20' of the zodiac. Your birth nakshatra (janma nakshatra) is where the Moon was when you were born.
Each nakshatra is split into four quarters called padas of 3°20'. The pada fine-tunes the nakshatra's qualities and is used for choosing baby names.
We compute the Moon's exact sidereal longitude at your birth with the JPL DE421 ephemeris and Lahiri ayanamsa, then find which of the 27 segments it falls in.
Yes. The Moon moves about 13° a day, so it can change nakshatra during a day. Check a birth certificate or ask family; an approximate time works.
Each nakshatra is ruled by one of nine planets (Ketu, Venus, Sun, Moon, Mars, Rahu, Jupiter, Saturn, Mercury). That lord starts your Vimshottari dasha sequence.