I'm not sure to understand your question. But I suppose you want to change the focus order on your elements?
So when you press Tab, your cursor/focus moves to the next element.
This order is set by the way you put your elements in the scene builder from top to bottom.
So in your Hierarchy menu, you will see something like this:
GridPane .. x ..
-- TextField (This element is the first one who retrieves focus)
-- TextField (When press Tab, this TextField get focus)
-- TextField (When press Tab again, this TextField get the focus)
If you want to do it with code, you can use the function setFocus().