For general spills and messes on your leather, use a mild soap and water on a soft lint-free cloth. Squirt the soap on the cloth and rinse it with water, wringing out well. Clean the couch where needed, and let the furniture air dry. If the leather is dusty, use a hand duster or vacuum hose to clean the sofa.
Don't rub a spill too hard or you can damage your sofa. Gently blot the wet spot with a cloth or paper towel until all of the moisture is gone. Unless there is a stain, this should take care of any wet spills.
Toothpaste will work on some stains. Squirt a little on, rub it in, and wipe it off gently. Another stain remover is hairspray. Rub it into the stain and wipe until the stain is gone. If you have an ink stain on your leather, use nail polish remover or alcohol. Rub the ink until the stain is gone, and dry it with a blow dryer.
For heavier cleaning of your leather couch, make a mixture yourself by combining one part white vinegar with two parts linseed oil. Rub the mixture into the couch, allow it to sit overnight, and polish it the next day with a soft cloth.