You can use the vSphere Web Client and vCenter Server to license an individual host.
For information about managing host licenses, see vCenter Server and Host Management.
You can set up bulk licensing using PowerCLI commands. Bulk licensing works for all ESXi hosts, but is especially useful for hosts provisioned with Auto Deploy. See Set Up Bulk Licensing.
You can also operate ESXi without licensing for a 60-day evaluation period, during which you can access the full ESXi feature set. See About ESXi Evaluation and Licensed Modes