Problems writing app that will communicate with gateway(newbie,last bit of project)

Hi all,

Im writing an app that will contact a specified gateway and retrieve info onto our server and perform manipulations on it then return the result back to the gateway for further operations...

But I'm a newbie and dont have much of an experience in network programming...
Would like to know what are the paramaters necessary to contact the gateway...(I've got permission to use a gateway from a certain provider.)

Could anyone please help me...(This is the last bit in my project , that im not too familiar with)
The protocol is probably going to be HTTPS...
We have a choice to choose between that and HTTP... working out the logistics as of now..

This is however the overview...
The gateway will be pushing text to our server, we maintain a DB to do the manipulations on the incoming data.
Process depending on our needs and then push the result back to the gateway which will then take it forward.

I'll be using Java ...
The gateway that will send the data to us will be in the form of an url, this will be processed by our server(Which will act as a client since the gateway now acts as server).
So should i be looking at writing a HTTP client app or servlets, because regular socket programming ,Im assuming will only be used for RAW TCP/IP packets..

Assume that the data is sent like this from the gateway to me.

http://some.domain.com/path?source=1234&destination=3456&uname=xyz&passwd=123

In this case all I need to do is to verify if the uname/passwd is valid or not and send a response back to the gateway(It will take if forward from there).
(i.e, I do a GET the url and then extract the query string that contains the necessary data.
Then I process the data with a DB that I have to check of certain things in the extracted data.
Then I POST the response from my server back to the gateway).

I dont have a Submit button or anything like that at all, so how am I going to use the POST method(to the gateway) if that be the case?(want to know what will be the argument to be passed to the POST so that it will take the data to the gateway)

The reason Im asking is so far I had only dealt with the POST method in the conventional way ...
This is new.

Could anyone help me out.

I'll be glad to help you out with more,if need be...
This is the last bit of my project and taking too much of time to resolve.
please help.