@erfan-shaigani
I can not write a Node-Red tutorial here - but there are a lot of videos.
I can recommend this site - to get familiar with Node-RED this page: http://www.steves-internet-guide.com/node-red-overview/
and his starter video: https://www.youtube.com/watch?v=y05vQUIMIxs&feature=youtu.be
instead of the mqtt-in node - you can use the "iobroker in" node.
So if you have open your node red - drag an "iobroker in" node into the flow area. Configure this node by opening this node - and select your datapoint of your temperature sensor - as you can see in my screen-shot.
Then wire for the first time a debug node to the "iobroker in" node and have the debug window open in node-red.
Everytime your temperature will change generates a new message, which you can see in the debug window.
With this message - containing the temperature you can do what you want. Create your json string, you can use to put it in an other data object, write to a file, a database, put in an http-request and even use the simple api.
io.png