Good Question, it's really quite easy. When you see your tama on the screen looking like it's crying or has its back to you go to the first icon on the bottom and select praise. If your tama beeps for no reason (check status bars) go to the same place but click Time Out instead.
-Also, when you wrap something up to give to another person (by connecting) and your tama gives another present (such as poo, nothing, a snake,..etc) he will come back into your tamagotchi and he will start laughing. Punish him, and you will have another bar full!