How do I know how much food I have stored in my home already.
The best way is to make an inventory. As you are going along look at what ingredients you have and try to figure out if you can make complete meals. The trick that I use is I tend to think of it like a camping trip. I leave my home with a plan. During my time away I have planned what I will eat for each meal, and hopefully when I return home I will come home with nothing left. This is how you create a plan and how you know how much food you have stored, it is the same principle.
Things are looking bad, what do I do first?
Start out slow and try to cover all of your bases. Get things that you need without trying to panic and get caught buying too much of one thing in particular. Remember the big picture and purchase lots of little stuff versus one big thing at a time.