I would assume 0 now, and add enough CYA to get to 30; it's a good place to start, and leaves room to add more later if you need to (like the option of leaving a floater with pucks in while on vacation!)
You can buy CYA (sometimes called stabilizer) at the pool store- or some Walmarts (mine doesn't carry it...)
You can put it in the skimmer- but don't backflush for several days, as it will collect in your filter and dissolve over time.
You can also put in a sock and hung in front of your returns.
After a week, test your CYA again, adjust more if needed!