1) scenarios
think thru what kinds of things can happen that you're interested in prepping for. some places it will be hurricanes, other places being snowed in 2 weeks; some people it will be job loss, other people it will be medical problems.
you'll probably get a long list, but don't panic, because 90% boils down to a few simple things -- water, food, protection from the elements, etc. for me, it ended up being storing food, and buying some camping gear (stove, sleeping bag, tent) to be prepared for utility outage or if i had to bug out for some reason.
2) goals
i started by setting myself a series of goals. first goal was to have 2 weeks worth of food/water in the house, then 3 months, then a year. i also had goals for other emergency supplies (eg, medical kits, camping supplies, etc.)
3) re: food
store what you eat and eat what you store. if you don't eat spam, then don't buy a case of it to store on your pantry shelf. look at various lists, but realize none will ever be an exact fit for your diet and your situation, so just use them for ideas.
for me, i looked at what was easy to store, and adjusted my diet as well. before i started prepping, i didn't eat beans very often, but dried beans store really well. so, i tried some recipes, and turns out i love black beans and garbanzo beans. who knew!

i was also trying to improve my diet and health, learn to cook more from scratch, so for me it was an evolving process. for others, they may not be interested in changing anything.
4) keep learning
much of prepping is really about knowledge, and finding alternative ways to do things. we americans have a tendency to want lots of gadgets to solve our problems, but often there is a simple solution if read and learn of how other creative people solved the problem, or how the problem was solved in the "old days" before convenience food and freezers.
5) start small and keep it simple/cheap at first
one of the first things i did was store 5 gallons of water in reused soda bottles. took neglible time and money to do it, but greatly improved my psychological peace of mind. today, i've got 5 gallons bottles and berkey water filter, but all that extra money only gives a small increment of capability over what i had with the simple refilled soda bottles. same with food -- you can get expensive MREs, or freeze-dried camping food, or you can buy beans and rice. there are a few advantages with the more expensive solutions, but don't overlook the simple, and don't do *nothing* because you think anything less than the most expensive top-of-the-line is worthless -- it's not!
--sgl