This is part of my complete guide to Setting up a CentOS Digital Ocean droplet with Nginx for beginners.

If you just need the commands, here they are. Read on for an explanation of what is happening.

sudo systemctl start firewalld
sudo firewall-cmd --permanent --add-service=ssh
sudo firewall-cmd --permanent --add-port=4444/tcp #if you changed default ssh port
sudo firewall-cmd --permanent --add-service=http
sudo firewall-cmd --permanent --add-service=https
sudo firewall-cmd --permanent --add-service=smtp
sudo firewall-cmd --reload
sudo systemctl enable firewalld

I am not going to bother repeating the excellent explanation of these commands from their source guide. If you are interested, check it out.