From Ubuntu 12.04 LTS, Apche have new version 2.4 which is different from 2.2 and below.
The configuration hierarchy is listed in apache2.conf file 's comment.

# /etc/apache2/
# |-- apache2.conf
# | `--  ports.conf
# |-- mods-enabled
# | |-- *.load
# | `-- *.conf
# |-- conf-enabled
# | `-- *.conf
`-- sites-enabled
#   `-- *.conf

# * apache2.conf is the main configuration file (this file). It puts the pieces
#   together by including all remaining configuration files when starting up the
#   web server.
# * ports.conf is always included from the main configuration file. It is
#   supposed to determine listening ports for incoming connections which can be
#   customized anytime.
# * Configuration files in the mods-enabled/, conf-enabled/ and sites-enabled/
#   directories contain particular configuration snippets which manage modules,
#   global configuration fragments, or virtual host configurations,
#   respectively.
#   They are activated by symlinking available configuration files from their
#   respective *-available/ counterparts. These should be managed by 127.using our
#   helpers a2enmod/a2dismod, a2ensite/a2dissite and a2enconf/a2disconf. See
#   their respective man pages for detailed information.
# * The binary is called apache2. Due to the use of environment variables, in
#   the default configuration, apache2 needs to be started/stopped with
#   /etc/init.d/apache2 or apache2ctl. Calling /usr/bin/apache2 directly will not
#   work with the default configuration.

I. How to change DocumentRoot in Apache 2.4

 I found that we need 3 steps:

1. Add more Directory  in  /etc/apache2/apache2.conf

<Directory YOUR_FOLDER>
 Options Indexes FollowSymLinks
 AllowOverride None
 Require all granted

2. Change default documentRoot in /etc/apache2/sites-available/000-default.conf :
   DocumentRoot /var/www


  DocumentRoot YOUR_FOLDER

3. Restart your Apache server : sudo service apache2 restart or  sudo /etc/init.d/apache2 restart

II. How to add virtualhost

We need 3 steps to complete this configure

 1. In /etc/apache2/sites-available folder  create SITENAME.conf file
( SITENAME is your name you want to set)
In SITENAME.conf  config your new virtual host
<VirtualHost *:80>
        ServerAdmin webmaster@localhost

        ErrorLog ${APACHE_LOG_DIR}/error.log

        CustomLog ${APACHE_LOG_DIR}/access.log combinedme


Remember : documentRoot should be configured before ServerName.

2. Run command : sudo a2ensite SITENAME.conf  
3. Restart your Apache server : sudo service apache2 restart or  sudo /etc/init.d/apache2 restart

※ Notes: Using debug command : bapache2ctl -S to get to know how your server is configured. 

You can see more how to config in this link