I suggest adding more info – What language are you coding it in? VB?, SQL?, C++?, JAVA? Is it a self contained database or just a front end for use with existing backend dB – SQL Server, Oracle, etc? Do you need just billing (invoicing) or will it need to maintain receivables as well? Does it tie into an available ERP / CRM system?
I strongly suggest starting with a data model. What data do you / the end users need? Obviously, to start, you’d have customer data (what fields?), Water Usages, Rates, etc. Map it all out. Then construct your tables, keeping data as Normalized as possible (This will allow for easier expansion, linking, etc).
For starting everytime you start your PC – Could it just be added to your Start-up folder (assuming Windows machine – you didn’t say).
Give us some more background…