How to create a registration dialog?

You can store the data in Sql server. To protect the data there are many ways. But you need just to access the data on the right way (protecting from sql injections). That means to use store procedures or parametrized commands. When you insert users, first questions is what field to be key for the user. One of mostly used is email adress which can be his username in your application. Storing password is a different story. You need to protect the password against any one even you. If by some fault security, some one reads the data, you should protect from getting users passwords in the application. Common solution is to encrypt the password with one way encrypting method which means that you can't decrypt encrypted password. Now there is also new encrypting model to raise encryption security, and that is to use Password Salt and Password Hash.