I can't log into my new MacBook Air

Did you bought this computer and the first thing it shows was a password and you did not set up a password? Then you most likely bought an activation locked Mac that is useless to you. Apple will only remove the lock if you have valid proof of original purchase information

If it is stuck on a password you've set up and you also don't have access to the recovery key, your only change could be to erase your Mac

If you still can't reset your password, erase your Mac

If no other solution is successful, you have the option to reset your password by erasing your Mac.

1. Shut down your Mac, then start up from macOS recovery as described previously.
2. When you're asked to select an admin user you know the password for, choose Erase Mac from the Recovery Assistant menu in the menu bar.
3. From the Erase Mac window, click Erase Mac, then click Erase Mac to confirm. 
4. If your Mac restarts to a flashing question mark, press and hold the power button for a few seconds until your Mac turns off.
5. Start up from macOS Recovery again, then reinstall macOS. For details, see How to reinstall macOS. 

If you can't reinstall macOS because the installer doesn't see a hard disk on which to install, you might need to change the format of the disk:

1. Press Command (⌘)-Q to quit the installer.
2. When you see the utilities window, select Disk Utility, then click Continue.
3. Select the first item listed in the sidebar of the Disk Utility window. This is your built-in hard disk.
4. Click the Erase button or tab on the right side of the window, then enter these details:

• • Name: Macintosh HD
  • Format: Mac OS Extended (Journaled)
  • Scheme (if shown): GUID Partition Map

1. Click Erase, then click Erase to confirm.
2. When the erase is complete, press Command-Q to quit Disk Utility and return to the utilities window. You should now be able to reinstall macOS successfully.

See this link for further reference

https://support.apple.com/en-us/HT212190