I would like to buy apple care

If the Apple Care was transferred to you from the previous owner, then you may be able to extend it via the Monthly option: link-> Extend your AppleCare+ coverage - Apple Support

If not, and if it has been more than 60 days since it was purchased from Apple, there is no way to add Apple Care to it now.

Unless the "someone" is an authorized Apple reseller and the product is brand new, you cannot buy Applecare on a used product. If the Mac is used and had/has unexpired Applecare, you can call Apple and make sure the records are changed so it shows under your devices. It will continue to be covered but only up to the original expiration date - you cannot extend it on a computer. At the moment Applecare plus can only be extended on an iPhone.