Do a Master-Detail scenario with a DataGrid and TextBox`s beside, how to bind to multiple selections?

Question

My user can Select multiple Orders in a OrderDataGrid from a selected customer. When the user selected the OrderId + Ordername in the DataGrid all Order details must be filled in the TextBoxes righthand of the Order datagrid.

The problem I have is how can I bind always to the first element of the selected orders? At the moment the TextBox binds to this:

That does not work :/