To begin with, when you place items on the wall, it is much easier if you use the Snap Furniture function - in this way the objects automatically 'stick' to a wall.
When you move an object along the wall (or anywhere in the room), you can see 4 arrows that show its distance from the four walls in the room. However, to specify an item's position on the vertical axis, you need to click on the i-button and open the Property Inspector.
The X / Y / Z axis determine the object's exact position in your plan. To specify on what distance from the floor you want to place an item, enter a value for Z. If Z=0, this basically means that the object is on the floor. You can also use the values for Z to place one object on top of another or for different compositions.