The most common modulations are modulations to closely related keys. There are several methods that we can use to find closely related keys.
In major keys, find the subdominant and dominant chords, their relative minor keys and the relative minor key of the main key.
In the case of C major:
In the case of a minor key, find the subdominant and dominant chords, their relative major keys and the relative major key of the main key.
You can also look for degrees II to VI of a major key:
Or III to VII degree of minor keys:
Closely related keys are neighbors in the circle of fifths: