How to create a server accessible by an iphone app

I a thinking of creating an iPhone/iOS app that would include a feature where one user could create a list of words and then save them to their account on a server. Also (and this is very important), the user could share their list with other users by giving them permission.

So my question is, how can I go about creating such a server? For right now, I have a home computer (running Windows XP that just stores data for my music system) which I can use to host the server. I am also open to the use of other online storage services like Google Drive or Dropbox (I can't remember if Amazon does anything like that). However (and I know this may complicate things a bit), but at least for now, I want/need to stick with free services/options.

Just to recap, the key features that I am looking for are:

create users/accounts (on the server)
- eventually I may [try] to incorporate the use of other services to log users in like with their email account, OpenId, etc.
the ability to access (log in to) the server (with credentials) from my app
the ability to send/receive data between the server and my app
the ability to share data between users

I know this is a lot to ask for, but if anyone has any suggestions or can get me going in the right direction, it would be much appreciated.

Solution

The basic setup would be as follows:

Backend: Database (MySQL), Web server (Apache), with server side scripting (PHP).
Client: iOS device with developed app.
Communication: use HTTP client/server model, communicating with something like JSON.

This is much the same setup as a web server, but instead of serving html/css/javascript etc the results will be JSON.

As far as implementing specifics such as login in, and sharing data between users, this is purely dependent on your implementation. This is not trivial, and not something that can be easily stated in a single post.

Hope this helps.