How to create Abap function

Hi Experts,

I would like to create a ABAP function that can take in value and update the value into the custom database table. How do i do it? Is there any step by step procedure material?

At the end of the day, i would like create a web service of the function and allow the developer using Interactive form to pass the value into the web service and update the database table. How do i accomplish that?

Your feedback is very much appreciated. Thanks

Regards,

Rayden