After installing vCenter it is time to add ESXi server, before adding ESXi Server there must be Virtual Datacenter created before hand, Add Esxi server inside Virtual datacenter. Open vSphere web client, right click vCenter server and click New Datacenter, Provide the name and click Ok.
Next right click newly created VMWare Datacenter, Add Host, type the Host name of esxi server (I have already one entry in DNS server for this esxi server and it is always a best practice to use FQDN), click next.
On the connection settings provide root username and password of esxi server, vCenter server will use this credentials to install its VPXA agent on the server. Also there is a SHA1 thumbprint of ESXi certificate is shown (If esxi server has self signed certificate, default one) once the connection made, Click yes.
This is Host Summary about Esxi version and hardware model, it also shows Virtual machines on Esxi, I don't have any at current.
Assign license if you have any, Currently I am running it in Evaluation trial, and it will expire in 60 days, I am planning to add one in future. Click next.
Lockdown mode is very interesting setting while adding host to vcenter server. Once lockdown mode to set Normal, Esxi can only be managed and administratored through vCenter or DCUI (Direct console user interface - local console), You will not able to login to ESXi directory using vSphere client or SSH/shell, it will be disabled. Although this can be disabled later by editing Security Profile in host settings.
Next setting is which folder VMs need to put, I don't have created any folder yet and they will located in default first virtual datacenter.
This is last check and click finish if everything is good on Ready to complete esxi summary page., you will see one task is created under current task.
Within few seconds new esxi host will be visible under vcenter > datacenter, now this ESXi is read for creation of VMs virtual machines.