Q: how could i erase everything from my macbook air

Maybe not the easiest, but if you have a USB Drive that you can clone with something like Carbon Copy Cloner, you can reboot from the USB Drive and then erase/format the MacBook Air.  Then reinstall.

You can erase the disk using OS X Recovery.

Start your Mac while holding down the Command + R keys. From there you should be able to access the built in utilities to reformat (erase) the disk then reinstall OS X using OS X Recovery.

That only reinstalls OS X, none of your personal data.

What to do before selling or giving away your Mac - Apple Support

backup with time machine

Use Time Machine to back up or restore your Mac - Apple Support

boot into recovery mode and erase the hard drive.

https://support.apple.com/en-us/HT201314

goodbye data.

then reinstall the OS from recovery mode.

however, what are you trying to do that you need to erase the drive?