Initial Handicap Index calculation (the first Handicap Index) requires at least three 18-hole or six 9-hole scores. In continental Europe (EGA) the minimum is one score of round over 18 holes or 9 holes.
The Rules of Handicapping describe the procedure – see Initial Handicap Index page.
Calculation of the Score Differential in such a round is subject to specific rules. Therefore it may be a bit complicated for less experienced golfers. So that a new calculator has recently been available:
Scorecard – Initial Handicap Index calculation
with a built-in Score Differential calculation mode
in a round played for Initial Handicap Index purposes.
If you are interested, just try it out.