how do I run a virus check for my macBook Air?

You would have to download an antivirus software. But, since there are no active virus for the Mac, the whole exercise would be futile.

At least ClamXav is free: https://www.clamxav.com

It's best to describe the problem, if there is one, in as much relevant detail as possible, rather than what you think is causing it or how you think it should be solved.