Salesforce Trigger to be Written

I am having the following scenario : I am having the object "Project " - API Name : pse__Proj__c On the Project I am having the field : Planned Hours - API name : pse__Planned_Hours__c I want that every time the Planned Hours field is edited or information is added in this field to ask for a note creation , like an error message to popup . I know that this is not achievable with Validation rules and I don't know how to write triggers. Can somebody help me? I Know this is something super simple but I don't know how to do it. I can pay 5 EUR for this task.