Your smoke alarm should be installed in the middle of the room, or at least 50 centimetres from any obstacle (as far as possible from the wall), with a minimum of one smoke alarm per floor (ideally one per room).
The best areas to install your smoke alarm, will be in hallways and stairways, close to or in each room. It is highly recommended to not place your smoke alarm in kitchens or bathrooms: smoke and vapours could cause false alarms.
Please also avoid garages, laundries, fireplaces where combustible particles could disturb your alarm, and all locations close to airstreams. In these areas its preferable to install thermal alarms instead.