As a beginner it may seem a bit taunting on how to actually transfer data, or how to ask the correct question on how to do it. The reason is because there are so many ways to do.
A popular way to be run a simple webserver on your "server". You can choose any language you like to run this simple webserver, python, node.js or C# MVC with API.
On the node, all you need to do is call the webserver on the specific URL.
You may want to use a WebAPI approach to make things simpler.
http://pi/alarm/on <---WebRequest----< Make a webrequest
http://pi/alarm/off <---WebRequest----< Make a webrequest
The code behind the server and the nodes is up to you. Pick a language you would like to learn, Python being the best documented with samples for Rasbpain and C# for IoT - With tons of documentation and examples too.
Once you learn the basics, you can add data to a request. Like the name of the node, timestamps, or even post photos/video as binary.