forgot startup password for macbook air. Please help.

Try one of methods on this page

Change or reset the password of a macOS user account - Apple Support

If you can't do any of these, then I'd suggest making a new administrator account. This will not delete any of the existing users. It's a little tricky to do, but just follow the steps.

Start your Mac up in Single User Mode. To do that, hold in Command+S as you turn ut on. Then write in the following:

mount -uw /

rm /var/db/.AppleSetupDone

shutdown -h now

Please note that Single User Mode is using an American keyboard layout which might not match the layout on your built-in keyboard.

Then start up your Mac again and you'll be met with the Setup Assistant. You'll follow this to create a administrator account.

Then go to the site I linked to and follow the lines where it says Reset using another admin account.

I hope this helps!

See is this helps reset your password - Apple Support