The first thing to know, and it changes how urgent the rest is: you already have some water, and it is in the pressure tank.
When the pump stops, whatever is in that tank is still under pressure and will come out of the taps until the pressure drops. It is not much: typically enough for a couple of toilet flushes and some hand washing - and it is why you get water for a few minutes after an outage and then nothing.
So the practical sequence when the power goes:
Fill everything immediately. Bath, pots, kettle, buckets. The first five minutes are the only free water you get, and most people spend them working out what has happened. Filling the bath alone covers toilet flushing for a day or two.
Know how many flushes you have. A toilet cistern can be refilled by hand from a bucket; that is what the bath water is for.
Then the longer-term arrangements, in ascending cost:
- Stored water for drinking and cooking. Cheapest by a distance.
- A generator sized for the pump.
- A hand pump or a well bucket, if your well allows it.
- A larger cistern, which some people on unreliable supply install.
Most households stop at the first two.