Summarize:

Start wiping down your furniture to remove any dirt and debris. Try wiping down the piece with a clean, slightly damp rag first. Then take a cleaning wipe like Lysol or Clorox and go back over it. You don't want any dust or grit to get stuck under your coats of paint. Before you start painting, be sure to remove any handles, buttons, or decorative items that you don't want to paint over. As you remove them, pay attention to where they went and how they were installed. This will make it easier when you have to put everything back on. Place each piece into a ziploc bag so they don't get lost after you start painting. If you are painting an older piece or one you purchased from a thrift store, be sure to check it for any marks. Look over the piece first for large discolorations, indents, or scratches. Then run your hands over the item just to be sure. If you find any, use a filler like Elmer's Wood Filler and a putty knife to close up the gouges. You can also leave these marks if you would prefer a more dated and worn look. Take a fine grit sandpaper like Pro Grade Precision 220 and use a sponge or sanding block for consistent pressure. This will keep your sanding even. Work from the top downward. Make sure to sand with the grain as sanding against it can deeply damage the wood. Vacuum off and wipe down the furniture again once you've finished.
Clean off your furniture. Remove any hardware. Fill in gouges and deep scratches when needed. Sand down wood furniture.