How do I remove all malware and viruses from my Macbook Air?

A few months ago I lended my laptop to my cousin and he, unkowingly, downloaded some viruses/malware, now I'm not sure how to get them out or what they're called. He thought that he removed it but when I'm on safari, I can be on one website and then I'm suddenly sent to another on a different tab - this happens quite frequently and I'm fed up of having to deal with it. My Macbook has also become slower due to this. Thank you, if you need any extra information about anything I'd be happy to give it to you.