[development] how to implement hook_views_data() and add a field?

Hi, All I want is just return a number, which comes from a sql query count of my custom table. I want to represent that as a field so I can adjust it's position in the views ui. It doesn't sounds like very hard, right? I've looked into the views api 2, but there's really no useful information... Anybody can help? Regards, Yi Yang