How to setup DDNS to automatically track your IP changes?
I have a Debian based systems, should I use apt-get install ddclient to install it?

Please enter the name of the server which is providing you with dynamic DNS service (example: members.dyndns.org
I don't know the answer to this question? when I try to install the ddclient?
should I fill in my IP's Domain?

See https://www.dynu.com/DynamicDNS/IPUpdateClient/DDClient

Look specifically at "Below is a sample configuration to detect IP address for third level hostname MYDOMAIN.DYNU.COM from the hardware interface"

I just did this minutes ago on my Raspberry Pi (Debian 9.8). I had to figure out how to get it to post my IP over Windscribe VPN. If you're in that boat too, make sure your /etc/ddclient.conf has

#use=if, if=wlan0 (my connection was over wireless)
use=web, web=checkip.dynu.com

Then restart the service using
sudo /etc/init.d/ddclient restart
then refresh your profile after a couple of minutes to see the change