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.