I would check whether it is an Intel Mac Mini or a Power PC one. If you look at the model number on the hardware you can probably check on the internet. Alternatively, plug it into a monitor and look in the dropdown menu below the Apple symbol in the top lefthand of the screen.
Intel hardware can take the latest Mac Operating System OSX 10:11. The Power PC can only take OSX 10:5 which is several years out of date. The reason I am asking this is because because you don't want to adopt and learn on an obsolete machine and OS.
Assuming it is Intel, the Mac is quite a lot different from a Windows PC but is easy and intuiative to learn.