How to type a Trigger

Hello everybody,
I have a problem with wirte a trigger. I try do describe situation. I have an ERP system where orders from customers are saved. Orders are saved via software that synchronizes with online stores. In these orders there is a "Comments" field where some information is saved. I would like the trigger to pull out one piece of information and save it in the appropriate field of this document. Let me give you an example. The following information is entered in the "Comments" field: Courier: DPD Document: Invoice Client: xxx I would like the word "Invoice" to appear in a specific field of this record while it is being saved. How to write such a trigger?

Thank You very much harishgg1. It's very helpful.