how can i connect to database?

I would suspect that the problem lies in your connection string. I would suggest you to use the element from your config file and setup your connection string there so you wont be declaring that in everytime when you connect to your database. Here's a quick example of setting up you connectionstring from the config:

     

Then you could query your data like this:

string connectionString = ConfigurationManager.ConnectionStrings["DBConnection"].ConnectionString;

DataTable dt = new DataTable();

    using(SqlConnection connection = new SqlConnection(connectionString)){

        connection.Open();

        string sql = "SELECT Field1, Field2 FROM YourTable";

        using(SqlCommand cmd = new SqlCommand(sql,connection)){

                SqlDataAdapter ad = new SqlDataAdapter(cmd);

                ad.Fill(dt);

        }

}

hello 

Please specify the error.Then we will help you.. thank u

    Öncelikle 2 tane kütüphanenin ekli olduguna emin olman lazim. Bunlar
    using System.Data.SqlClient; ve Usint System.Data;

    Daha sonra sql'in içerisine veri tabanini çikardigina emin ol ve veri tabaninin ismine emin     ol. Sql açildigindaki ekrandaki Server adi, baglanacagin Veritabani adini kontrol et çünkü o       bilgileri kodun içine yazacaksin 

       baglanti.ConnectionString = "Data Source = COMPUTER1; database =NORTHWND; integrated security = true";

     bu kisima. Eger pasword alanin bos sa integrated security yerine Trusted_Connection =          true yaz.

        Aldigin hatayi tam olarak yazarsan daha net olur...
        bu koddan anlasilabilecek bisey yok.
        Söyleyebilecegim kütüphaneleri ekledigine emin ol. Server,database i kontrol et sifreni kontrol et