Find the shut-off valves
If you're on mains water, the most important thing you need to do is find out where the water shut-off valve is and how to turn it off.
If you have the older type of earthenware drain, tree roots could be creeping into the cracks. If you notice the water level in the toilet pan oscillating, or gurgling noises coming from basin and sinks, you may have a partial blockage. A Master Plumber or Drainlayer can unblock them for you. Newer PVC drains rarely get blocked, so you might want to consider replacing them at some stage.