To use WhatsApp, first download and install the app from the App Store (iPhone) or Google Play Store (Android). Open it, verify your phone number, and set up your profile with a name and picture. You can then start chats by selecting contacts, send text messages, voice notes, photos, or videos, make voice and video calls, and create groups to chat with multiple people at once. WhatsApp also offers status updates and end-to-end encryption for secure messaging.