What tithi was I born on?
A tithi is a lunar day, and it does not line up with a civil one — it can begin mid-afternoon and end the next evening. So the tithi you were born under is the one that was running at the moment of your birth, which is often not the tithi a calendar prints against that date.
What this tells you, and what it does not
It tells you the calendrical state at one moment: the tithi, its paksha, the lunar month and the nakshatra. That is a calendar calculation, and it is all this page does.
It does not cast a chart, describe your character, predict anything, or rate a day as auspicious. Nor does it work out when your tithi birthday falls — that depends on which convention a household follows for choosing the civil day, and we would rather publish that once it is settled than guess at it here.