I made a project with c#.net and I am using sql server database. So what I think I have to do when I copy the app to my client's PC is to install sql server express and attach my database to it, but now I am having a problem.

I am worried that the user can insert data using the sql server instead of the app or he can play with passwords and important data. Can anyone tell me how I can secure my database or how to lock the server with a password, so no one but me can enter it? thanks