You should have at least one smoke alarm on each floor of the house, and one within 3 metres of each bedroom (or alternatively one in each bedroom). The smoke alarms also need to be working. Test this by holding down the "test" button. If no (or a weak) sound comes out then the battery needs to be replaced.
Check that the smoke alarm is on the ceiling or high up the wall. Avoid placing smoke alarms too near corners of ceilings and walls as this can be a "dead zone" where the smoke doesn't get to.
A photoelectric, long-life smoke alarm. These detect heat from fires as well as smoke and are less prone to false alarms. Ensure they are placed on the ceiling or high up a wall, but not too close to the edge of the wall or ceiling.
Ionising smoke alarm. These detect smoke only, have batteries that need replacing annually and should be replaced with long-life photoelectric alarms.