Paint doesn’t stick very well to dirty walls, so clean them with soap and water (or TSP) and rinse with water once done. Let dry overnight.
Clean ceilings before painting. There will be cobwebs and dust that you just don’t see. Use an old paintbrush or china bristle brush to dust baseboards, trim and crown molding before taping them off (use painter’s tape with paper attached to protect the baseboards). Overlap the tape seams by at least an inch to avoid seepage between pieces, and then seal the tape to the surface with your fingers or a rag.
Don’t cover the floors with plastic as it can be very slippery. Use drop cloths or old sheets to protect them from splatters and drips.
The color mixer at the paint store can create slight variations from can to can. The last thing you want is to have two shades of a color on the same wall. This can be remedied by mixing all of the paint cans together before you start painting to ensure you will have consistent color throughout the room.