This is an excellent do-it-yourself project!
A good stiff brush with a good grout cleaner works well and can be found at any professional flooring wholesaler or retailing outlet. Use a mild solution of 10 parts water to one part phosphoric acid. Fill a bucket with this solution and use a stiff nylon brush to scrub the joints clean. The phosphoric acid solution will splatter and get on your clothes, cabinets, and wall coverings, so be sure to wash all surfaces down with water when you are done.
Once you have cleaned the grout, seal it with a grout stain. Purchase a stain in the color you want for the finished grout. Apply it with a brush and then wipe it off the surface of the tile. Stains color the grout, make it look brand new, and is the best sealer you can buy.