Assuming you already have shelter, the order is pretty much Water, then Food, then Security.
WATER - isn't just about having a lot of stored water, but also where to find it, and how to have a replenishable supply. Rain barrels are great ideas, so may want to check into these. They can be a little pricey, around $100 for a store bought one, but there are other, more second-hand solutions for these. Even having some stored bottled water though, can be helpful. Remember to rotate it out every 6 months though....to keep it fresh.
FOOD - Food safe buckets with air-tight lids, and use mylar bags, then store things like rice, dried beans, mashed potato flakes, dried corn, etc. These items will keep for over a decade if stored right. And, you can build up.
That is really the key. You don't have to get everything at once. Start with a bug out bag, a backpack of supplies that you can take with you, keep in the car, etc. Then keep adding things. You will eventually be stocked up, just may take longer on a budget.