Which topic is the most crucial depends on how you’re defining it.
Most of the time, sanitation and clean water are going to be huge problems. I tend to lump them together because they’re closely related.
But, if someone is just starting to dip their toe into the prepper world, what I tell them to start with is a 3-day power outage. Why? Because it gets them thinking about things without overwhelming them. Once they’ve prepared for a 3-day outage, and have done a test run by turning off the power for a weekend, their brain will keep working, expanding on things, and before they know it they’ve changed their whole outlook.
Got your 2 topics noted - water & sanitation and helping others.
That’s a great idea to start out thinking about a short amount of time. A lot less stressful and I’m sure a lot of ideas come out of it.