The song Something Fishy has two chords, the C chord and the F chord. Since there is a C note in both chords, you can keep your fifth finger on it when you change between them. This will make changing chords very easy.
This song contains all three chords you have learned so far. Remember to keep your fifth finger on the C key when changing between C and F, and your thumb on the G key when changing between C and G7.