You can install Ubuntu server without difficult, go to http://www.ubuntu.com/download/ and must localize for the last LTS (Long Time Support) ISO file.
The ISO file let you make a bootable DVD or USB Key.
For this tutorial I going to use Virtual Box. I do assume you know how to use Virtual Box and I will not give details about configuration of this software
Step 1: Mount the Ubuntu Server ISO File
You must boot the VM from the ISO file, to do this, you should mount the ISO file into CD/DVD device of the VM.
For this step you have a recently created virtual machine for Ubuntu 64 bits with 1 or 2 GB of RAM and 20 GB of Hard Disk. To mount the ISO file on CD/DVD open the setting window of this VM.
Select the "Storage" option and the click on the CD logo with "empty" label. (first image)
On the right side, click on the small CD logo, on the drop down menu select "Chose Virtual Optical Disk File" and you must open the Ubuntu ISO File you download. (second image)
When the ISO file has been mounted (third image) just click the OK button.
Step 2: Start Ubuntu Server Installation
Star the Ubuntu Server Virtual Machine and the Ubuntu Server Install screen will show you some options
Click on Install Ubuntu
The screen will let you chose the language and press enter.
Step 3: Configure Language and Other Settings
The installer will ask you for language for installation, location, locales options (language), and keyboard layout. You can press enter for default options or personalize what you want.
Step 4: Configure Network Interface
The installer will tray to configure the network interface through DHCP and assign automatically an IP address, but if you are need a fixed IP adderess you should cancel the DHCP detection process.
Select "Configure network manually" and enter the IP Address, Netmask and Gateway. Select "Continue" with <TAB> key and press Enter.
Step 5: Configure Hostname and Other Options
Now you should give a host name, domain name, full user name, user name, and password.
Please enter the data on each value and select "continue" using the <TAB> key and press enter.
On the case you are asked to encrypt your home directory, chose "No"
The last screen on this step will let you chose your time zone. Select your time zone with up and down, press the <TAB> key and press Enter to continue
Step 6: Select Disk to Installal
Select second option and press continue, accept default options for next screens.
The last screen of this section respond "yes" to write changes to disk. (look second screen)
Step 7: Select Software to Install
Next screen configure proxy if you need one to connect to internet, other wise just press continue.
Select the an option to Automatic Updates, I recommend you "No Automatic Updates".
Select the software to install. Mark "OpenSSH Server" and "LAMP Server", user space bar and up an down key to do it.
Step 8: Configure Database and Finish Install
Set a password for Mysql root user.
Select the place to install the boot loader (just respond Yes to the questions)
The installation is complete juts select continue to restart the system and boot from Ubuntu Server.
Step 9: Open Your New Web Server Running on Ubuntu
Now just open a web browser and type the IP Address of the new ubuntu server.
If all its working you will see something like this image.