UFW is the default firewall configuration tool on Ubuntu and is simple to install on Debian.

https://www.digitalocean.com/community/tutorials/ufw-essentials-common-firewall-rules-and-commands

Getting Started: https://www.howtoforge.com/tutorial/ufw-uncomplicated-firewall-on-ubuntu-15-04/

HowTo: https://www.digitalocean.com/community/tutorials/how-to-setup-a-firewall-with-ufw-on-an-ubuntu-and-debian-cloud-server

ufw --force disable
ufw --force reset
ufw default deny incoming
ufw default allow outgoing
ufw allow 2222/tcp                #Example
ufw --force enable

ufw status