One of the easiest ways to divide your house into two is making a stud partition wall. You can choose from either adding a doorway or constructing a plain wall.
With this, you can decide to build the stud partition wall from the joists below or place it directly on the floor board. This makes the wall independent in case you need to lift boards a later date.
You could also set the partition sides against a plaster surface so that there is much easier job fixing with the masonry.
You could either decide to have this done by a professional but if you are good with the required tools; you could also do it yourself. If you decide the latter; make sure to have the right measurements and to have the all the needed tools.
Whether you decide to go with a professional or do it on your own; make sure you have the right knowledge of what comes with Stud Partition Walls.