Server

What do you mean full control of user PCs?  You mean you want it to be a domain controller?  And what do you mean web caching... For users browsing or your web site?

Even when you answer these questions I REALLY HOPE YOU UNDERSTAND - we don't know your current systems and how you work and utilize them... the only way to get an accurate answer is to analyze the performance of your existing systems, determine where your bottlenecks are, and address them specifically in a new server.  Further, would you be virtualizing?  (You should, but that will also affect your specifications).

Hi,
All I have cisco router>>>cisco switch (core)>>> to different departments.
All switches are connected to core switch by fiber cable.

Yes I want all users to be in domain controller.

Thanks

So you don't have ANY existing servers?

No I don't have any server.

Thanks

I would recommend a Dell or HP Server, a Dell R610 or HP DL360G8.

These are rackmount servers, do you have a rack?

Web Caching?  Perhaps you mean DNS chaching?  Server by itself is not a proxy server and will not hold web pages in memory to speed up web access in the way that Proxy Server or ISA would.  There are programs that will do that, but Server alone will not.

For 100 users.  What else will be on this server?  SQL?  Accounting?  File and Print Services?  If you want a Domain Controller you should consider if Server 2012 is compatible with your main applications, assuming you will install some there.  If it is not, then you will have to drop back to the version that is compatible, which will require open licensing.

For hardware:  HP/Intel/Dell, two Xeon processors, all the RAM you are able to budget, (where 8-16 (imo)is the minimum, no need to go crazy here;  take into consideration the requirements of the other services you might run) and sufficient hard drives to provide the amount of storage you need in fault tolerance, where RAID1, 5, 6, 10, 50, 60 would be the choices.  Do not under any circumstances use just a single drive or RAID 0