In order to create a small network that will allow 12 computers to communicate you be glad to know you actually need very little equipment.
You have 2 directions you can go in though so first thing is you have to make decision!! I will give you some options for both though so no need to worry So first of all then, do you want to go wired or wireless when it comes to your network?
When making this decision you need to take into account things like: what the cabling is like in the building these PCs are going to be in, do you need to run in cabling for a wired network or does it already exist? Is wireless suitable from a security point of view? Cost always come into it at some point etc.
So for wired.
I am going to make the assumption that all your computers already have wired NICs (network interface cards). It would be quite unusual for them not to have
So the first thing you need to purchase is an ethernet switch. Being Cisco trained I would say go for something like a Catalyst 2960 if you are buying new, or depending on your budget look on eBay for a used Catalyst 2950 or 3550. But basically though you will need a port per device on your switch and so you will need 12 ports (1 for each PC) and 1 port for your ADSL router (when you get it). A 24 port switch will give you everything you need and room for expansion.
I would suggest that you bought your ADSL router in advance and connected that up to. The reason I am saying this is that you can configure it to become the DHCP server for your network and thus eliminating the need to configure each device with a static IP address. I would also make sure that which ever router you pick it has a firewall feature set. ADSL routers made by Linksys (now Cisco) and NetGear have always been pretty well priced and super easy to configure.
Add to that some cables and plug it all up, it really is that easy!!
(Afraid I cannot comment on cables though not knowing the building and whether or not structured cabling already exists)
So for wireless
This is slightly easier in some respects but does come with other headaches like security and performance restraints. Linksys do a great little box that does everything you need. Have a look at this: Dual-Band Wireless-N ADSL2+ Modem Gigabit Router (WAG320N). In addition you would just need to specify some adaptor for your computers. have a look at the following link and pick the ones most suitable to your PCs.
I hope this helps!