VMWare vCenter Server is a commercial product and is not available for free download. The options you have are:
1. Get an evaluation license: You can request an evaluation license from VMWare which will give you a 60 day trial of vCenter. You will need a VMWare account to request an evaluation license.
2.Use an open source alternative: There are some open source alternatives to vCenter like oVirt and Proxmox VE. They provide similar functionalities for free.
3. Build a lab environment: You can build your own lab environment with ESXi (VMWare's hypervisor) and use the free Standard version of vCenter Server to manage it. But the Standard version is limited to 1 CPU and 4GB RAM.
4.Consider the VMWare Cloud Foundation license: This gives you vCenter and other VMWare products for free, but requires deploying it on VMWare's infrastructure ( hence the name Cloud Foundation).