The idea of "being prepared" can involve so many things in so many different situations. The key is to ask yourself what you cannot, or do not want to, live without. For us, the answer is water.
Whether you live off-grid or in the city, having rain water catchment and a tank is not a bad idea, if only as a backup. Most urban water reaches the home using electricity, so if the power fails, so does your source of water. Furthermore, there are times when your pipes can freeze, again cutting off your water. If you have a tank on your property, you can, at the very least, go and manually get a bucket of water for general sanitation.
When providing your own water system - whether as your main supply, a backup or for fish or irrigation - the bulk of your efforts should go towards storage, or a tank, as that is the most costly part.
Plastic tanks are easy to set up, but they are very expensive in larger sizes. Concrete ones are durable, but ill advised for any beginner, as they are prone to leaks if not done correctly, especially where the walls and floor meet. We have found that the shell and liner system described in this article is by far the best option for us. It's cheap, easy (even for just two people), and quick.
For information about how to harvest the rain from your roof to fill your tank, see here
. For further information on cisterns, click here
. For the full article, click here