Ufw is for managing a Linux firewall and aims to provide an easy interface to iptables for the user. Install and configure ufw do this as root user or use sudo:
apt install ufw
Set default rules
ufw default deny incoming ufw default allow outgoing
Allow connections
These commands both allow a connection on port 22 using the TCP protocol. Use one they end up in the same IPtables:
ufw allow ssh ufw allow 22/tcp