I'm a new beginner in the express environment and i've one question(i know that it's probably a stupid question): once i've created my own app written in node.js, where can i upload this app?
It sounds like you're basically trying to figure out how to deploy a Node application so you can use it publicly.
Good question.
I think the simplest way (and this is just one of many) is to use Heroku. I'm not affiliated with them at all.
Once you've signed up for Heroku, you can essentially deploy your Node app to their service using just a few command line commands:
$ heroku create
$ git push heroku master
$ heroku open
This will essentially create a new Herkou app, deploy your application there, then open your browser to the newly deployed app page so you can test it out.
Heroku is a hosting provider that lets you deploy apps using Git, and handles all of the dependencies, etc.