What the sankalpa is
Every ritual opens with a sankalpa — a formal statement of intent. It locates the ritual precisely: the cosmic era, the year, the ayana, the season, the month, the paksha, the tithi, the weekday, the nakshatra, the place, and then the person, named with their gotra and the ritual's purpose.
Gotra
A gotra is a patrilineal descent line traced to one of the ancient rishis — Bharadwaja, Kashyapa, Vasishta, Vishwamitra, Atri, Gautama, Jamadagni, Agastya and others. It is inherited from the father. A woman takes her husband's gotra at marriage in most traditions.
Pravara
The pravara is the list of specific rishis in the line, usually three or five names, recited after the gotra. Families often know the gotra but not the pravara; a purohit familiar with your tradition can supply the standard pravara for a given gotra.
If your gotra is unknown
This is common in families who moved generations ago. The accepted practice is to use Kashyapa gotra, on the reasoning that all lineages ultimately trace to Kashyapa. No ritual is invalidated by this. Tell your purohit at booking so they are not searching for the information on the day.
Where to find it
Ask the eldest living member on your father's side. It may also appear on old wedding invitations, in family records, or on a temple's family register if your family has one.