Question about Selection Objects


This is My first post and I need help with a small project that i do.
I trying to Create Selection Static Actor System like old game Theme Hospital: (First Stage Building a room (blue background)) i creating RTS system Selection, but this is not good it is not precocious.

I think i need get coordinates from first Actor i click and then get proper draw position, anyone can tell with function i need to use to achieve it?
pic below



You need a raycast for the selection and Mouse Screen Position for UI draw.