I am using WPF toolkit provided DataGrid control to display product list along with its OpenStock, Description etc. In this DataGrid i have set OpenStock column to editable
You need to set the current cell to the one you want edited and then call BeginEdit in your Loaded handler:
dataGrid1.CurrentCell = new DataGridCellInfo(
dataGrid1.Items[0], dataGrid1.Columns[3]);
dataGrid1.BeginEdit();
If you give your DataGridTextColumn a name in XAML you can use that name rather than Columns[3].