How do I put anti virus on new laptop

[Lottiebugz22]

Do I need to buy a disk?

Most new PC's come with built in anti virus software. If you were to buy AV software it would be a digital download, not a disk. Do you even have a disk drive on your new laptop?

Most new PC's come with a free trial of an antivirus software.
In order for the software to continue to work after the trial you need to purchase a "key" online.
If you want a different antivirus to the one installed then you will need to uninstall the old software first.
Antivirus is a thing you should only have one of.

It's a ASUS Chromebook C223 11.6

As it is a Chromebook and not a Windows OS laptop, there is already a lot of built in protection due to the way Chrome "sandboxes" everything and uses "verified boot".
You can still add extra protection if you want, it's not going to harm the machine Putting on something like Malwarebytes will check any new app installation (as does the Chrome OS itself). Bear in mind any free app will have adverts. Use Google Play to add and install anything as the apps from there are verified.
Make sure the machine is kept up to date with the service packs from Google