I saw a house recently that has wave pattern on the ceiling and paper on walls. What would I need to do to make ceiling flat and remove the paper to paint it?
There are many approaches. One of the least labor-intensive (but folks who have not gone down the route of removing wallpaper, etc. almost never believe it) is to apply a new layer of drywall (thinnest you can source) over the walls & ceiling you don't like the paper/texture on. That spares you from hours of steaming wall paper and fighting with the resulting wall (usually not) taking paint nicely, and filling and/or scraping off the ceiling texture. You may need to extend electrical boxes, but you get a nice new paintable surface without the fuss of removing the old one.
Or, if electrical or insulation upgrades would be good anyway, rip out the existing drywall and replace it.
Of course, if you "saw a house" the easiest approach is to not buy that one. If you buy it anyway, letting it be for a year to see if you still hate it enough to bother is often worthwhile.