How to calculate required resistance?
The simplest manifestation of reducing voltage is called a voltage divider, and requires two resistors to implement.

Typically the resistors are wired in series with one leg (call this R1) connected to a source voltage (call this V_in) and the other leg (call this R2) connected to ground.

In this configuration, the voltage at the midpoint of the divider, between the two resistors, under no load, is definitively equal to: V_in * R2 / (R1 + R2).

Pick R1 to be a standard resistor value, then solve for R2. The values you pick will determine how much steady current is demanded by the divider, namely: V_in / (R1 + R2).

If that output voltage is under load, you have to do something like run it through a non-inverting buffer (e.g. Op-amp) before using the voltage.