There are two different types of treatment for sick fish, and the one to use depends upon which illness your fish has. Here is the description of the diseases (from the Fish Tycoon instructions):

Fish from Isola tend to get two types of diseases: fungus looks like a greenish-brown growth on the fish; ick looks like whitish specks on the fish. Both of these diseases are bad for your fish as they can spread to other fish and eventually kill them. Just purchase the correct medicine from the supply shop, then drag it from the tool box into the tank.

Most of us don't treat sick fish in the beginning, when we don't have much money. You can either sell them or trash them. There are some pretty good "getting started" tips in other posts on this forum. You should check them out, too, for additional help!