There are three types of modulation. Most modulations occur between closely related keys which are those keys that differ by no more than one accidental in the key signature and their relative minors. (see chart)
G A B C D E F G
E F G A B C D E
C D E F G A B C
CM: I ii iii IV V vi vii0 I
GM: IV V vi vii0 I ii iii IV