Hi and Welcome,
If you are referring to a Magic mouse then as there is only one button, it knows whether
( Its clever little devil) you want a right click but only if it senses a single finger on the right of the mouse. Anything else is considered a primary click.
But if you are referring to the Mac trackpad then you set this up via system preferences - trackpad - point and click - second option - secondary click. (As my screen shot)
But it should be pre set so just gently tap the pad with two closed fingers and you should then get a drop down menu ( Its clever little devil to
). Hope this helps.