By button, do you mean the icon menu? There's helpful guides online, such as this, this and this. They're all quite similar, so just have a look and see (depending on what Mix you have, i.e Spacy, Melody, Anniversary, Sanrio).
Otherwise, are you talking about the three buttons on the Tamagotchi itself? The one on the left is the A button, which is selection/scroll. The one in the middle ( B ) is to execute/confirm, and the one on the right ( C ) is to cancel.