How to Retrieve value from a database or dataset

The article link given by Uday P.Singh is good for learning about ADO.NET.
Now, if you have already populated the DataTables of your DataSet, then to retrieve a DataRow corresponding to the ID entered in a TextBox the following code can be used.

//Assuming Employees is the DataTable, EmployeeID, Name and Address as columns of this table
//If the EmployeeID is the primary key in the table, the Find method
//of DataRowCollection returned by Rows property of DataTable can be used
DataRow row = Employees.Rows.Find(TextBoxID.Text);
if (row != null){
    TextBoxName.Text = (string)row["Name"];
    TextBoxAddress.Text = (string)row["Address"];
}
//If the EmployeeID is not the primary key in the table, then Select method
//of DataTable can be used, which returns an array of DataRows matching
//the filter expression given as the first parameter.
DataRow[] rows = Employees.Select(string.Format("EmployeeID='{0}'",
                    TextBoxID.Text),"",DataViewRowState.CurrentRows);
if (rows.Length > 0){
    TextBoxName.Text = (string)rows[0]["Name"];
    TextBoxAddress.Text = (string)rows[0]["Address"];
}