As the root user or using sudo run the following command:
apt-get install php-apc
Everything else is done for you and all that is needed now is to gracefully reload Apache, again running as the root user or using sudo:
The configuration file is at /etc/php5/conf.d/apc.ini should you wish to change any of the settings such as the APC memory size etc.
If you want to have a page showing the current APC statistics, run the following command:
gzip -dc /usr/share/doc/php-apc/apc.php.gz > /path/to/apc.php
Make sure /path/to/apc.php is a location available in a web browser (and ideally password and/or IP address restricted) and then you can view detailed statistical information. I have some screenshots available here from an older post.