how do i erase all info on macbook air while still keeping microsoft word
Your best bet for a clean slate would be to wipe the drive and reinstall OS X. However, if you really need to keep Microsoft Word you might want to try other options.

For data that the old owner did not delete, you can create yourself an account on the machine (Administrator) and delete their account. 



Then, using Disk Utility, you can "erase free space" to actually delete the data.



As for the rest of the stuff on the drive, such as installed applications that you don't want, you will need to manually remove these from /Applications. I suggest that you don't simply delete the .app files from that directory, but run them through a 3rd party uninstaller such as AppCleaner.

This is a lot less "clean" than reinstalling OS X, but it does what you want.