If you suddenly, for some reason, you decided to sell your MacBook Air or to give to use anyone for a while, you will definitely need to clear some data from your account. As competently and quickly do this read this in this instruction.
First of all, you will need to make a new account. Name him "admin" or something like that and come up with a simple password. From this account you can delete personal data and files from your own account, as well as other user credentials. This method is quite simple, but it can take quite a long time, up to 10-12 hours. If you do not rush anywhere, then this solution is quite suitable for you. Faster method is the formatting of HDD or SSD. Do not be lazy for complete security to make a backup for the contents of the computer. Format the disk and install the appropriate operating system.
The new owner will receive a computer without unnecessary files that do not need him, and all your data will be safe.