Securing a Linux machine is always a good idea, and here's the simplest way to do it.
First, install ufw and check the status. It should be disabled by default.
apt-get install ufw ufw status
Second, we will enable the default rules which will be to block all incoming and allow all outgoing.
ufw default deny incoming ufw default allow outgoing
Now for specific ports to open. Here I will allow SSH (22), HTTP (80) and HTTPS (443), webmin (10000) and seafile (8000)
ufw allow ssh ufw allow 80/tcp ufw allow 443/tcp ufw allow 8000/tcp ufw allow 10000/tcp
Now we enable it, and then check the status.
ufw enable ufw status
To disable it.
To reset back to default
And that's it, all incoming traffic is blocked except what has been explicitly allowed, and all outgoing traffic is allowed.