This tutorial will show you several ways on how to redirect an URL without any www at the beginning.
For example, you go to:

mywebsite.com

You'll be redirected to:

www.mywebsite.com

I'll show you how to do this using .htaccess file, and then using conf files.

Redirection using .htaccess

1. Go to your www/ directory.
2. Create or open a .htaccess file.
3. Type this, replacing the example domain name by yours:

   RewriteEngine on
    
   RewriteCond %{HTTP_HOST}                !^www\.mywebsite\.com(:80)?$
   RewriteRule ^/(.*)                      http://www\.mywebsite\.com/$1 [L,R=301]

4. This will do a 301 Redirection, which is the HTTP code for "301 Permanently moved". So that you don't get a duplicate site on Google (one with www, one without).
5. No need to restart Apache.
6. If this does not work, try the method using conf files.

Redirection using *.conf files

1. Go to your apache directory (might be /etc/apache2, or otherwise try 'find / -type d -name 'apache2' to search for it).
2. You'll have to look into those files to find your virtual host configuration:

   /etc/apache2/httpd.conf
   /etc/apache2/sites-available/default
   /etc/apache2/sites-available/*.conf

3. When you have it, you should see something like this (this can be a lot bigger sometimes):

   <VirtualHost 218.0.101.14:80>
           DocumentRoot    /home/mywebsite/www
           ServerName      mywebsite.com
           ServerAlias     www.mywebsite.com
           <Directory "/home/mywebsite/www">
                   Order Deny,Allow
                   Allow from all
                   AllowOverride All
           </Directory>
    
   </VirtualHost>

4. Add those lines, before the </VirtualHost> tag:

   RewriteEngine on
    
   RewriteCond %{HTTP_HOST}                !^www\.mywebsite\.com(:80)?$
   RewriteRule ^/(.*)                      http://www\.mywebsite\.com/$1 [L,R=301]

5. This should look like this:

   <VirtualHost 218.0.101.14:80>
           DocumentRoot    /home/mywebsite/www
           ServerName      mywebsite.com
           ServerAlias     www.mywebsite.com
           <Directory "/home/mywebsite/www">
                   Order Deny,Allow
                   Allow from all
                   AllowOverride All
           </Directory>
    
   RewriteEngine on
    
   RewriteCond %{HTTP_HOST}                !^www\.mywebsite\.com(:80)?$
   RewriteRule ^/(.*)                      http://www\.mywebsite\.com/$1 [L,R=301]
    
   </VirtualHost>

6. Save, and restart apache.

   sudo /etc/init.d/apache2 restart

7. This should do it!