learning virtualization and want to try to set one up

so I am completely new to this, but I am interested in setting up a server (a Dell T20) up as a NAS, Plex, and web app development server. My limited understanding of VMs is that they run separate OS's that can be purposed for different needs? So I could run FreeNAS on one VM and another linux OS (haven't figured out what's easiest/best) for web stuff?

I was looking into ESXi vSphere6 and didn't realize the vCenter is not free. Do I even need vCenter?

This is all very overwhelming so any help is greaty appreciated.