first of all sorry If I don't write properly, I have got a bad english.
Well I have got these servers:

The server number 1 has got so much work so I want to setup a cluster system joining 4 virtual machines.
I don't know how to start, at this moment I have got servers 3, 4, 5 and 6 only with the OS.
Which cluster software could I use?
Can I install apache and mysql normally on the VMs? or I must install a specific version of these software?