well, you're not supposed to just turn it off if you actually want it to grow and such...As Honahlee said, you can pause it, but this will also stop all growth. If you want it to grow quickly, try to avoid pausing as much as possible. If you've never raised a tamagotchi before, I suggest reading the instructions, as you will learn so much from them :]
If you don't have time to care for it and want to turn it off for a while, stick the plastic tab back in or take the batteries out.