The most important thing you should have in your backup supplies is drinkable water. Most prep blogs will tell you that — and in the next sentence move on to which 1911 frame handgun you should buy (rolled eyes).
Don’t waste money on boxes of the stuff in sealed Mylar bags, that’s where scammers grift the prep community.
If you have space, the blue 55 gallon storage barrels are the way to go. Be sure to buy new ones if possible and certainly food grade. The white ports (bungs) on the top are slightly different; one has fine threads and the other has coarse. The fine threaded one accepts a hand pump so before filling, set your barrel up so that port is not against the wall.
Preservation is not difficult. You can play the diluted bleach game (more on that in a future post), toss a piece of silver in there (according to stories) or use two part Aqua Mira additive for 5 years of security and be done with it. That’s what I use and you can see the bottles lined up.
Cycling is necessary. I mark mine for 4 years and now use a submersible 12VDC pump (green in the photo) to drain them out. In previous houses I summoned the power of Grayskull to He-Man those 400 lb. beasts to the edge of the garage to tip over.
If you do not have room to put a barrel in the corner of your garage then 1 gallon plastic bottles are your easy path forward. Water is 8.3 lb/gal so they are not too hard to shift around.
The important thing is that if you have none right now, do something today. Don’t overthink it or wait for the “more perfect” solution; excellent can easily become the enemy of good enough. Good enough will keep you alive.