Day 1 of sourdough baking is making the "sponge" or preferment.
To begin, you need a sourdough starter. You can buy one, get one from a friend, or make your own pretty easily. If you keep it in the refrigerator, you'll need to get it to room temperature first (consider that Day 0).
Remove one cup of your starter and toss it in a mixing bowl. To this, add:
10 ounces bread flour
16 ounces warm water
(I use weight to measure for greater consistency. If you don't have a kitchen scale, this is about 2 cups unsifted flour and 1 3/4 cups water. However, I recommend you get a scale, you'll end up with much better baked goods).
Stir it up until fairly smooth. Depending on how wet your starter was, the weather, and how humid it is in your kitchen, you may need more or less moisture. You want a wet, sticky concoction that looks something like this:
Cover with plastic wrap and let it hang out in the fridge overnight. I usually make my sponge at about 4 or 5 PM, but a couple of hours either way isn't going to affect things much.
Go on to Day 2