My MacBook Air says maleware has been detected. How do I get rid of it?

Without knowing where you are getting that message it is hard to answer but I’m going to make a guess based on previous experience. I suspect you were browsing the internet when you got this message. If so, ignore it, it was a phishing attempt to get you to click on a link that would give you malware. If my suspicion is wrong, provide more information please.

It's a scam, but as dwb wrote more information would help to determine the cause so that you can stop the annoyance.

Please refer to Virus notification on MacBook Pro. - Apple Community. If that describes what you are seeing it also describes how to correct it.