It's different to wpf.
You do it like:
var selection = reb.Document.Selection;
selection.StartPosition = 1;
selection.EndPosition = 2;
reb.Focus(FocusState.Keyboard);
Where reb is your richeditbox.
That's selecting one letter of course.
For position 0:
reb.Document.SetText(Windows.UI.Text.TextSetOptions.None, "abcdefghijklmnopqr");
var selection = reb.Document.Selection;
selection.StartPosition = 0;
selection.EndPosition = 0;
reb.Focus(FocusState.Keyboard);