I was just working on an app that involves polling and sending messages to the server for leader board.
1) You definitely need a server with server side scripting and database install. PHP and mysql are probably the most popular and cheapest in term of price..
2) You need to setup server side script to read your data input from AGK. Have you worked with this before?
3) AppGameKit has native functions such as HTTP. AppGameKit does a good job of explain this in their documentation.
//add stats to server
http = CreateHTTPConnection()
SetHTTPHost( http, URL_BASE, 0 )
//some codes....
SendHTTPRequestASync( http, urlSuffix, qryString)
while GetHTTPResponseReady(http) = 0
Sync()
endwhile
response$ = GetHTTPResponse(http)
CloseHTTPConnection(http)
DeleteHTTPConnection(http)
4) You pass data from AppGameKit to server via query string in any format you like. Ex: JSON, raw text, etc
5) You can retrieve data from AppGameKit via response$.
6) If you want it to be secure, there are paid and free SSL installation for servers.