OK.  Once you’ve downloaded Ubuntu the next decision will be where to install it. My suggestion is go virtual.  In the past I ran Linux on my work laptop and on my personal iMac; both using VMware Fusion.  As of this writing I am running v8.5 and the current version for Mac is v10.  The other way of going is to install VMware ESXi on a server and create a Linux virtual machine.  Perhaps the easiest way to get started using virtualization is via Oracle VirtualBox.  That’s a great way of learning not only about Linux but also virtualization.