If you are using SQL Express (which you are), then your login credentials are .\SQLEXPRESS
Here is the connectionString in the web config file which you can add:
<connectionStrings>
<add connectionString="Server=localhost\SQLEXPRESS;Database=yourDBName;Initial Catalog= yourDBName;Integrated Security=true" name="nametoCallBy" providerName="System.Data.SqlClient"/>
</connectionStrings>
Place is just above the system.web tag.
Then you can call it by:
connString = ConfigurationManager.ConnectionStrings["nametoCallBy"].ConnectionString;