Dividing/separating rooms?

Question:im going to move into a new place and it was advertised as 3 bedroom, it was really 2 bedroom. one downstairs, and the master upstairs, and an office space as you get to the top of the stairs , leading towards the master bedroom..
i need to convert half of that room into a room and make the rest of it a hallway to get to both the master and the new bedroom.


how can i make this open room into a real room and a hallway leading into both rooms.

Answers:
We really need to know the dimensions of the area. They do sell some cheap software for home design that might help.