Unfortunately, the newest version of OS X released by Apple for your model Mac is Lion (10.7.5). You can purchase Lion from Apple for 19.99 USD. However the website states you first have to have Snow Leopard (OS X 10.6.8) installed before installing Lion. AFAIK, Snow leopard is no longer available for purchase from Apple. It is possible to buy used copies of Snow Leopard and it is also possible to download free copies. Downloading a free copy would violate Apple's license.
All of this really does not matter, unless you have some really old OS X compatible software to use. Basically, Lion is to old to access the web and be compatible with any current software.
Officially, the most current version of Windows available for your model Mac is 32 bit Windows 7. However, you might be able to run 64 bit Windows 10, but with a processor speed under 2 GHZ and only 2 GB of RAM, I would expect Windows 10 would be sluggish to run.