Since marble is extremely porous, it stains easily. Don't even think of using a marble surface in a kitchen or bathroom until it's been treated with a penetrating stone sealer. A marble tabletop on which you serve food should be sealed as well. Depending on the wear and tear a marble surface undergoes, you may need to re-apply the sealer as frequently as every six months to ensure the surface remains resilient to stains.
Wipe down marble counters, tabletops, and walls rountinely with a clean cloth or a sponge dipped in warm water and mild liquid detergent or stone soap, and dry the surface with a soft cloth. Be careful not to use too much soap, and rinse thoroughly, because soap can leave a film on marble and cause streaking.