How do I make a bread starter from scratch and how long does it take before I can use it?

You can go the traditional sourdough route, but if you have a bit of dry yeast to jump start the process, you can create a pre-ferment dough that you can use as soon as 12 hours after mixing. There are various versions, known as a biga, poolish, or old dough, or probably other names I can't think of now. Erin McDowell here at Food52 did a nice bit about them, https://food52.com/blog/17140-preferments-and-how-to-adapt-any-bread-recipe-to-use-one
It also includes some recipes to get you started. I'd recommend you start there.

Hi Sally! To make a sourdough starter from scratch all you need is good-quality all-purpose flour and filtered water. In about 1 week, you should have a starter that's ready for bread. Here's the recipe: https://food52.com/recipes/80659-a-simple-sourdough-starter I hope that helps!