Update & Upgrade RPI.

sudo apt-get update
sudo apt-get upgrade

Install Mosquitto mqtt broker and Subscriber

sudo apt-get install mosquitto -y
sudo apt-get install mosquitto-clients -y

Open Mosquitto.conf File

sudo nano /etc/mosquitto/mosquitto.conf

Delete the following line in the file opened

include_dir /etc/mosquitto/conf.d

Paste the following lines to replace the deleted line

allow_anonymous false
password_file /etc/mosquitto/pwfile
listener 1883

save&exit the nano editor

Now you can add your username with this command. Type in your username in the section called "username".

sudo mosquitto_passwd -c /etc/mosquitto/pwfile "username"

For example like this.

sudo mosquitto_passwd -c /etc/mosquitto/pwfile fatih

For more detailed information, see this video : https://youtu.be/_uFVRrQsGKU