How do I delete an app (Skype) on my MacBook Air?

Let me Google that for you:

http://lmgtfy.com/?q=unistall+skype+from+Osx

First result:

How can I completely uninstall, and then reinstall Skype for Mac OS X?

https://support.skype.com/en/faq/FA12073/how-can-i-completely-uninstall-and-then -reinstall-skype-for-mac-os-x

Drag it to the Trash; empty the Trash. You could also remove any Skype folders from (user)/Library/Application Support, and (user)/Library/Preferences/com.skype.skype.plist though these just sit there and don't do anything without the application.