Have a good look at the walls, will any need sanding down? You'll need to do this first to prep the walls for painting. You'll know if they need sanding as they'll be cracked or something similar.
If they're in good condition, clean the walls, apply an undercoat, then apply the paint. Make sure you cut in first along the skirting. Have plenty of ventilation as you're pg. Ask at the paint store for best paint ideas and painting tips.