The first step in any collapse scenario is civil unrest and a break down of law and order. You need to be able to survive the immediate threat to your life that will occur. It's like when you're camping - your first priority should be shelter because you can live for weeks without food or days without water but the cold can kill you in minutes.

In a full on irreversible collapse that leads to the end of the petrochemical industry, there will be no more fertilizer. Without synthetic fertilizer, there won't be enough food to feed everyone who is alive today. Estimates are that 2 billion people will die, but it's probably more than that because a breakdown in transportation would also make markets less efficient with local shortages all over. So you'd have to survive up to a 50% cull that would take place over a year or so.

On the more sensible range of things, small scale disasters happen all the time. Hurricanes, ice storms, power grid failures, tsunamis, riots, the draft, you name it. So being able to eat when there's no power for a week, or chasing off a mob or two will get you through the most likely scenarios.