A SQL code for create login and user then design it to my database.

Start by designing the DB to hold it: you need to hash your passwords, and that has an effect on storage. See here: Password Storage: How to do it.[^]
After that, all you need to do it design a login page and process the entries to check if the user should be logged in.

We can't help you with that: "C#" doesn't give any information on the environmnet the app is to run under, and that affects a whole load of things. For example, a Winforms app will be different to a console app, which will be different from a WPF app. A website will be totally different and need very special handling (and the use of built in access control features to make it reliable).