If you have the appliance configured, you can do this:
Log in to the VMware vCenter Server Appliance Web console.
On the vCenter Server tab, select Status.
Click Start vCenter.
If you still need to setup the appliance you'll find the documentation here - http://pubs.vmware.com/vsphere-50/index.jsp?topic=/com.vmware.vsphere.vcenterhost.doc_50/GUID-0B9988....
After deploying the vCenter appliance OVF and assign network from the console, Connect to the appliance IP via a browser, go to Services tab. From there we can start the vCenter service.