Assuming you are talking about an outdoor concrete surface that is heavily soiled:
Tools: broom, hose, bucket, scrub brush, trisodium phosphate (TSP), power washer (optional)
1. Sweep the concrete surface with a broom to remove any loose debris.
2. Wet the surface with a hose and allow it to soak for a few minutes.
3. Fill a bucket with water and TSP and mix according to package directions.
4. Scrub the concrete surface with a scrub brush dipped in the TSP solution.
5. Rinse well with clean water from the hose.
6. Allow the surface to dry completely before walking on it or applying any sealer or paint (if desired).