Specifically you will need to know: With that information an astrologer can calculate which side of the Capricorn Aquarius cusp you were born on, and therefore which sign you are.
For example, this calculation is always performed as part of my natal readings (for one person) and relationship readings (for couples).
The cusp between Capricorn and Aquarius happens at a precise moment, which varies from year to year.
Capricorn is an Earth Sign and Aquarius is an Air Sign.
Capricorn thrives on organization and is always looking for consequences.
Aquarius is modern and finds routine troublesome and boring.
If you don't know your birth time then things get a little more complicated, but it is often possible to figure it out "backwards" from major life events.
That's a technique called birth time rectification.