Go to petco or petsmart get a 10-20 gallon tank. Buy a heater, a simple HOB filter and test kits for Ammonia and Nitrates. Either buy more mixed water or make some up for water changes, your going to need to do water changes.
What type of fish and how large, is the fish still eating, are your sure it's marine ich?
Welcome to reef2reef! Lots of good info on here. Look through the disease forum here for some info. If the fish was in a display with other fish it is recommend to remove all fish and qt for 7-10 weeks leaving your display tank fishless. This is to rid your display of ich so it is not a reoccurring problem.