Placement of server-side code

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SharePoint

Hi guys I don't know how to start developing an app on sharepoint. I have the server right now installed and I used the templates, which were helpful but I have some requirements that must be deployed.

Where should I add the code to be implemented on the sever?

Llittle description:
My work is on "Evaluation" between employees that should be calculated
-each employee will evaluate all his/her colleges
-the manager will evaluate all his employees
-the result to each employee is:
The average of colleges evaluations (50% weight) + the manager evaluation (50% weight)

Thank you very much for the help.