Hi,
Javascript is kind of dynamic/typeless language, so you just declare a plain array. Something like:
// declare the array
var vms = []; // or alternatively -> var vms = new Array()
// add element(s) to it
var vm1 = Server.findForType("VC:VirtualMachine", "<some-vm-id>");
vms.push(vm1);
// access the array or individual elements
System.log("First element: " + vms[0]);
System.log("Array of VMs: " + vms);