I can't place a block in Minecraft on my MacBook Air?

I just got a mabook air and started playing Minecraft on it. My little sister has an hp and she told me to right click to place a block, but when i tried it, it just destroyed a block. And i don't know how to use a bow and arrow either

A single (left) click will destroy a block. A two finger (right) click will place a block.

When playing Minecraft press the Escape key (esc - top left corner of the keyboard), click on the Options button, click on the Settings button, click on the Controls button and you can set the controls to be whatever you want them to be.

You can go into the settings menu and enable the secondary click.