Start with the Apple menu -> About this Mac -> More Info (I think it was still called "More Info"
This should run the System Profiler (which is stored in Applications -> Utilities -> System Profiler)
Under "Hardware" the "Model Identifier" will give something like MacBookN,N
As well as your "Serial Number"
Plug the serial number into the following web site
<http://www.appleserialnumberinfo.com/Desktop/index.php>
It will display detailed information about your Mac when it was manufactured, including an "Operating System and Software" that will specify the Maximum operating system version your Mac will support.
I'm going to guess that you can run Snow Leopard 10.6, but you should check to be sure. If it says "The Latest", then that would be El Capitan 10.11 (then again, my old white Macbook will ONLY go to Lion 10.7, which is why I suspect your Macbook will at least do Snow Leopard).
And if you want to go beyond Snow Leopard, you will still need Snow Leopard to get any Mac OS X version beyond Snow Leopard.
If the web site indicates you can run Snow Leopard (or a version newer than Snow Leopard), then you can order Snow Leopard
<http://www.apple.com/shop/product/MC573Z/A/mac-os-x-106-snow-leopard>