For those you enjoy the fish hobby, it's always exciting to take home that new fish. Assuming you've done your research and the fish you are adding to your tank is compatible with your current inhabitants, the next step to making sure your fish is healthy is properly acclimating the fish to the tank.
The best thing to do when you get a new fish, is to put it in a "quarantine" tank for two weeks. This is a totally separate tank with no other fish, but with the same water parameters as the main tank. However, it is often difficult for many fish keepers to have an extra quarantine tank hanging around.
Any pet store that is half-way decent will put your fish in a plastic bag. Take the bag and float it in your tank for at least 15 - 20 minutes. This will allow the water that is in the bag to become the same temperature as the tank water.
The next step is to get a bucket and a fish net. Open the bag, and pour the fish out in a net over the bucket, so that the water in the bag doesn't get into the tank. This helps prevent any impurities in the tank water from the the store from getting into your tank. Drop the fish in your tank, and watch it swim about!