Very sparse question – we probably need much more info for a great answer – so I will try for a good answer.
A – if 5 nework locally is 5 client machines and one server, then you need a 8 port switch to connect them and the appropriate cabling between systems and switch.
B – “how I add different clients” – from each workstation logged in as local administrator “Join the domain”.
C – minimum hardware is totally determined by the applications and traffic you will be running. I have old Proliant 800 with dual PIII cpu’s, 1 GB ram, small raid (4-17GB Scsi drives as 52GB array) using Intel Pro100 Server NIC (i960) and serving as primary login for 100 users.
a – the primary login server needs a good NIC and enough memory for concurrent sessions.
b – CPU speed is not near as important.
c – drive space only matters if it is the only server in the domain. Storage appliances mean most DCs should not be used for file servers.
That should get you started – Good luck.