What is wrong with the fish?
Most vets don't know much about fish so unless you are taking it to a specialist fish vet, you will probably be wasting your money.
If you do have to transport it, a large container of water or large plastic bag that is 1/3 water and 2/3 air. Eskies/ coolers are good and have a lid to reduce splash.
If possible have a battery powered air pump to blow air into the container while you are travelling to and from the vet. This will help keep the oxygen levels up.
Don't feed the fish for 24 hours before taking it unless the vet says to. If you feed it, the fish usually poop in the container and that can cause ammonia problems, which can stress or kill the fish.
If the weather is warm, have the car's air-conditioner on when the fish is in the car.