Your friend's phone number must be in your phone's address book in order for you to start a chat with them in WhatsApp.
If you cannot see your friend in WhatsApp, please follow these steps:
Contacts in your phone's address book that have WhatsApp will be displayed in the Favorites list (Contacts tab on Android). In WhatsApp, you can only start a chat with these contacts.
If you still do not see your friend in your WhatsApp list, it is possible that your friend does not have WhatsApp. It's easy to invite your friends to WhatsApp. Learn how to here.
Not sure how to get WhatsApp on your phone? Read this article to get started.
WhatsApp Support Team