If you have enabled iCloud on your iPhone as well as signed into iCloud on your Mac the setup process is seamless. If not here are the steps To setup Facetime
Firstly on your iPhone check that tou are signed in to iCloud and Facetime by going to Settings-> Facetime
Then on the mac computer of the person who wants to call you let them open up Facetime under Applications
Provide them either your mobile phone number or your email address that is visible under the Facetime settings you saw on your iPhone.
If the person does not have an icloud account they will be asked to create on.
You will now be able to call each other with video if both are connected to the internet over any wifi connection.