Red spots. PIR Sensors
Blue spots. Door/window sensor
Green. Smoke detector

When deciding on sensor locations, it's all a question of priority and of course, budget.

In the example on the left, First three PIRs would be located in:
1. Kitchen/family/meals.
2. Entry/gallery
3. Lounge
(In an older style 3 bedroom home with hallway then the third sensor would be located in the lounge covering entry)
Next PIRs would be in room with valuables such as main bedroom study or garage or outside sheds with tools etc.

If using door/window sensors, priority locations are rear and side where intruders are generally out of sight of neighbours. You can arm these while home.
One of the advantages of door/window sensors is the alarm and dialer is activated before intruders are actually inside.