If you are not much familiar with VPS technology managment then I would suggest to upgrade to managed VPS server with cPanel/WHM installed.
There are many VPS providers available in market providing cheap VPS with cPanel/WHM installed.
For once, I would agree with "thewebhosting" in as much that it would be a good idea to get some kind of assistance managing your server.
Your VPS provider may provide "managed" service for a fee where they take care of the system updates, issues, and administrative management tasks while you focus on mainly just the web hosting accounts you host.
If your VPS provider doesn't offer this service or it's limited like most, you can either move to a VPS provider that does offer these services or hire a third party company to manage your server for you. Incidentally, we do both of these things I mentioned here at my company where I am typing this message at as does PlatinumServerM and a few other of other regular Cpanel members around here who we can each help you get things set up properly and well managed, self included.
I would recommend whoever you get to help you though, that you probably stick to the service of someone you find right here in these forums not because I'd be happy to make a profit myself, sure, but rather because I personally know many of the guys around here providing management services and I know most who are here are not ones to take your money and never touch your server which is a common problem across the professional server management industry. There are a lot of people out there who know less about server administration than their clients who setup a "server management" service just to get the monthly fees which works up until their clients run into real problems.
Now with that said, let's talk about your original question ....
"How to Install Cpanel / WHM on a VPS Server"
First, I am surprised that you didn't order Cpanel when you ordered your VPS server as many VPS providers offer that as an option already installed. I know we certainly do and there are only a few who don't.
Second, make sure that your VPS server has enough resources to be able to run Cpanel. You really need an ABSOLUTE MINIMUM of 512 MB of memory to run Cpanel and that is going to be somewhat flaky and not really recommended. I strongly recommend servers with 1 GB minimum for Cpanel unless you are ready for some real headaches!
Third, preparing your server. Before you install Cpanel, it helps to setup the basics to get a good compile environment at least and get your server up to date with the latest patches and software updates:
# yum install selinux coreutils binutils make dialog gcc gcc-* glib*
# yum install libexi* libjpe* libpng* gifl* freetype curl curl-* xmlrpc
# yum upgrade kernel*
# yum update
> install CPAN
> reload CPAN
Once you got the basics, time to install Cpanel / WHM:
# wget http://layer1.cpanel.net/cpanel-universal-install.sea
# chmod 700 ./cpanel-universal-install.sea
(The above will take you 30 minutes to an hour or so to finish)
After you have the base install done, it would be a good idea to run the 'checkperlmodules' a couple of times at least until it runs clean without trying to compile new modules so that you get all the required Perl modules you need to run Cpanel properly installed on your server:
(Typically needs to be run about 3 times till it runs clean)
Once you have Cpanel installed, connect to WHM and go through the initial setup wizard to setup your server hostname, licensing, DNS servers, IP addresses, etc:
http://(your server primary IP):2086/
That's the basic rundown!
At this point, you will have a basic copy of Cpanel installed but it won't be configured or optimized and your system won't have any security hardening, configurations, or optimizations either. To do that properly, you will have another couple hours of work ahead of you if you are experienced (a couple of days if you aren't) to get things fully setup and working properly the way they should.