Be logical and methodical. Make sure you understand the workings of individual components you will be using.
1) Make a list of all the sensors and output devices you are going to use (cameras, mics, switches, displays, sounders, etc).
2) Get hold of the specifications/datasheets for all those, so that you know what input/output signals (current/voltage/frequency) they require/provide.
3) Get a microcontroller with enough input/output capability to handle all those sensors and devices.
4) Design and build any interface circuits needed to interface the micro to the sensors/devices.
5) Decide exactly what you want the whole system to do and program the micro accordingly.