For ChangeLog setup, you need to go on Administration->Change Log Setup and mark 'Change Log Activated' field. Then, you need to press Setup->Tables button. Now you have list of all tables in your NAV database. You can configure to follow Log Insertion, Log Modification or Log Deletion.
You can configure all of them or only one or two methods or tables. If you want to follow all table data, you need to set 'All Fields', but if you want only few data, you need to set 'Some Fields' and after that to press three dots button. You must to mark fields you want to follow. This is in the same time answer on your second question: you can follow complete table or only few fields.
When you finish with setup, you can follow logs in Administration->Change Log. You can see all configured changes in table, export in Excel, or print report.
I think, you can understand this fields, there are intuitive.
