Before starting, you need to go to the People/Contacts section of Outlook. You can access this via the icon below the Calendar icon on the left, as indicated in the screenshot below:

Adding contacts
-
Click New Contact at the top
-
Fill in as much detail as you want (such as name, email, physical addresses, or phone numbers) then then click Save at the bottom.

Editing contacts
- Click on a contact
- Click Edit at the top (or right click on the contact and choose Edit).
- Make any changes as needed then click Save