I have been told to install VMWare ESX on the Solaris server, but I could not find resources about this issue on the web, how is that possible?
VMWare ESX is a bare metal hypervisor installed from the ground up on Intel architecture machines.
If we are talking about Solaris for Intel, we are talking about VmWare ESX on the physical machine itself where now Solaris runs being reinstalled witj VmWare ESX (probably, I would double check).
Otherwise, Solaris for Intel can be installed on a VM in a VMWare ESX server.
Side note: If the question is about an old Sparc Solaris, that does not support VmWare.