apt is the package manager used by Debian, Ubuntu and other Linux distributions based on them. If apt-get does not exist you are likely using a different Linux distribution.

You can usually find out what distribution you are running by checking the issue file, you can do this with the command:

cat /etc/issue

If this is CentOS, RedHat or Fedora then you will have to use the yum package manager instead.

Instead of the command you listed, you could possibly use

yum install haproxy

There are likely to be other differences so I would recommend finding a tutorial specifically for your distribution. This guide on HowtoForge covers installation in CentOS 7.