Node.js web request to external server

I'm working on a nodejs project, and I've found something that I can't afford. In a single moment, my nodejs application needs to ask something to another webserver. This web is a javascript application. It receives an string and answers with another string.

How can I implement this in nodejs, or which modules do I need.

The schema is something like this

Nodejs --------> http://"webserver"/app.html?var=string

^                                      |
|_______________string2________________| 

you can issue HTTP requests by this package - https://www.npmjs.org/package/request or without it - http://nodejs.org/api/http.html#http_http_request_options_callback

You can do a request using the HTTP module or use the Request Package to simplify things (Available using NPM). Since these methods use callbacks like everything else in Node.js, you can easily carry on handling new incoming requests and only reply to that specific request once you received a response from the remote server.