Placement of objects

If you want to place an object (e.g. furniture), hold it in your hands (so you see the small object icon in the top right corner of your screen), or more precisely, move it into one of the quickslots/hotbar (this can be done in your inventory [key i]) and equip it (use keys 1-5 or your mouse wheel to change your item). You should see a preview of your object now when aiming at the ground. Hold your right mouse button to place the object