You have have heard the slogan: preparation is 99% of painting. It is.
If the prep work isn’t right, you can’t do a good job.
Make sure your walls are cleaned and washed.
Make sure everything is spackled — nail holes, etc. If you have any cracks in the wall, fix them first.
Don’t want to sand?
For something minor, no need to sand, just take a wet sponge to it. Wait for the spackle to dry, then use the sponge just like it were sandpaper. It will take the rough edge off without getting dust all over the house.
Now you’re ready to prime and paint.