Just bought a used Mac and I’m not able to download yosemity on it

This site provides more information. Best for older machines. Note, serial number is sent in the clear.

"A serial number is a unique, identifying number or group of numbers and letters assigned to an individual piece of hardware or software. It's used for various things depending on the product / brand but what is your Mac's serial number for and more importantly... what is it hiding and what can it do for you ?"

http://www.appleserialnumberinfo.com/Desktop/index.php

or

This site provides more information, but lacks security too.

"A serial number is a unique, identifying number or group of numbers and letters assigned to an individual piece of hardware or software. It's used for various things depending on the product / brand but what is your Mac's serial number for and more importantly... what is it hiding and what can it do for you ?"

http://www.appleserialnumberinfo.com/Desktop/index.php

http://www.chipmunk.nl/klantenservice/applemodel.html

( hint by K Shaffer )

How old is the Mac?

It may be old enough that you cannot install Yosemite on it, but you really haven't given us enough detail as to what is failing.