Back up your files with Time Machine or another backup method
Sign out of iCloud
Sign out of iMessage
Sign out of FaceTime
Open System Settings or System Preferences
Go to General
Select Transfer or Reset
Choose Erase All Content and Settings
Follow the on-screen prompts
Enter your Mac password if asked
Confirm the erase process
Wait for the MacBook Air to restart and reset
If needed, restart in Recovery Mode
Choose Disk Utility
Select the startup disk
Click Erase
Return to Recovery Mode main menu
Choose Reinstall macOS
Follow the on-screen setup steps