In order to add a connection string to the app.config for a .Net application you only need the following websites:

• To learn the format of XML node in the app.config: Storing and Retrieving Connection Strings
• To learn the format of the connection string: www.connectionstrings.com

E.g for connection to a SQL 2008 server:

< ?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <connectionstrings>
    <clear />
    <add name ="Default" providerName ="System.Data.SqlClient"
    connectionString ="Data Source=myServerAddress;Initial Catalog=myDataBase;User Id=myUsername;Password=myPassword;"
  </connectionStrings>
</add></connectionstrings></configuration>

However, you should not forget about security Securing Connection Strings.