What do I need to do to prepare to remove a textured (popcorn) walls?
As noted in the comments, you * NEED * to be positive that this isn't 70s vintage, because of the possibility of asbestos. If this was my house, I'd send a sample to a lab.

What's behind the wall depends on the vintage of the house itself. If it's post-50s, it's probably a manufactured board product like drywall. Prior to that, it's apt to be some kind of lath/plaster. Either way, you'll have some sort of a substrate to work from. Don't expect it to be in perfect shape after you've removed the texture -- you'll probably have to skim the whole thing. (This is where the comment about adding a new layer of thin drywall starts to look pretty good, though trim and electrical boxes need to figure into the calculation...)

Mask-wise, you should probably wait until you can procure a mask. This emergency isn't going to last forever in the way that you want your lungs to last forever.

When actually scraping, a spray bottle is probably the best option.

Paint layers on texture will make it much harder to scrape, as the moisture you're applying won't seep in as much. A sharp scraper will help, which usually means either a lot of blades, or the ability to quickly and easily sharpen them yourself throughout the day.