well, when i was single and young, i started prepping. I didnt know it was called that, but i shopped at Sams club and always had bulk purchases in my basement, because it was cheaper that way.
when i started getting serious a few years ago and wifey was on board, i had her make a regular shopping list, except I doubled it and left out the fresh food. we did that once a month for a few months till i had a months supply. I suggest you do it that way as long as you can afford it. if not, scale back and only get a few things extra and take your time. dont go broke or buy on credit to get a few extra cans of beans. BTW, beans!! green beans are low in calories and take up space, where as baked beans are high in calories and fiber and take up the same amount of space. we love green beans, but only store a few cases and grow a ton in our garden. I have a metric shit ton of condensed soup and raviollis, PB+j, high fiber a caloric content beans of many types etc etc etc..... bottled water for quick access to water. i have other sources for water and a filter too.
I dont just store food. I have extra clothes, boots, underwear, duct tape, tools, window plastic, tarps, work gloves, fuel, etc. I have whole pile of books that im not real interested in reading, but if there was no TV or Inet, im sure they would be fun.
once you get ahead and you have more than one item and you use it, replace it right away.