Don’t have any idea how to prep for painting walls. How should I start?
Give them a cleaning, then lightly sand them with a drywall sander. wipe off the dust, then you're ready to prime and paint
Remove any picture hooks or nails, fill any holes with filler then lightly sand. If you have any loose paint sand it. For wall paper you could use wallpaper stripper or apply hot water with a cloth and scrape off.
Before prepping a room for painting, protect the furniture and flooring against splattering paint or accidents.
Sounds like a lot of work but it goes fast. I use a swifter on my walls when I get ready to paint.
Good luck and have fun, wear old clothes LOL
The only thing I have to add to the great advice you have is to use a Swiffer, they are great for cleaning the walls.