I removed 3 layers of wallpaper in my 1950 house and found that apparently the original builder hung wallpaper instead of painting this bedroom. The walls have a gritty sand like texture and I don't know what to do in order to get them ready to paint and have them look nice
Re: Plaster Walls
If your old plaster is just ugly, and not damaged, your solution is pretty simple, depending on what you want your final look to be.
If there are holes, divots or cracks, you will need to repair those . You may also still have some wallpaper glue on the walls. It would be good to get off as much of that as you can.
If no repairs are needed, and you aren't crazy about the gritty sand texture (who is?), one solution would be to skim coat the walls and go for a smooth painted finish. (You can even skim coat over the glue if you are careful.) This is do-able if you have a good system for skimcoating.
You could also apply a new texture directly over the sand texture you have now, or wait and put on the new texture on a skimcoated surface.
Here's a site with lots of DIY information about plaster repair, texturizing, and skim coat applications.