macOS High Sierra’s Contacts app automatically creates a contact card for you—called your me card—when you set up your Mac. Information on your card, such as your name, email address, or picture (if you add one), can be used in other apps, such as Mail or Safari. If you add a nickname to your card, Siri uses your nickname instead of your first name.
To see your card, choose Card > Go to My Card. In the contacts list, the Me icon next to your name identifies your card.
To make a different card your card, select a card, then choose Card > Make This My Card. To change information on your card (or another card), select a contact, click Edit at the bottom of the window.
To make your card private, choose Contacts > Preferences, click vCard, then select “Enable private me card.”
To choose fields on your private me card to share or export, select your card, click Edit near the bottom of the window, then select the Share checkboxes for the fields you want to include when you share or export your card.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)