Pausing is very easy. Simply press button A and while still holding button A, hold down button B. The screen should have a picture of your tamagotchi and under it, it should say PAUSE.
Or, if that doesn't work, probably because you have a non-pausing tamagotchi (eg. any Japanese Tamagotchi), go to the clock and press button A then while still pressing button A, hold down button C. The clock should be the same clock, but w/ the word SET. While on set, your tamagotchi is paused.