The care for the fish is going to differ depending on what type of fish it is. If it's saltwater, I really suggest against it as getting used to caring for saltwater tanks can take some time and it would be best to start off with the tank first to get used to keeping the salinity at the right amount.
And many fish are called sharks but are really fish.
There's the Bala Shark
http://www.aqua-fish.net/imgs/fish/bala-shark-profile.jpg
These do well in groups and are omnivores. But they also grow big so aren't really good for home aquariums. Although sometimes in some cases, fish won't outgrow smaller aquariums, but as always the larger, the better.
Then theres the iridescent shark which is really a catfish
http://www.scotcat.com/images/p_hypophthalmus3.jpg
These also do best in groups since they are a schooling type. Again these will outgrow home aquariums and can actually grow to 3 feet.
Or the rainbow shark again, not a true shark but a bony fish
http://en.wikipedia.org/wiki/File:Rainbow_Shark_Fransenlipper.jpg
These guys tend to be territorial with the same species but can live with other species of fish. It seems that they do best in bigger aquariums.
There are other types, but these are commonly sold. As you can see, many are not actually sharks, as I think that would be really hard to care for as sharks need huge tanks being that they don't breathe like fish do. They have to be moving in order to breathe. So a large tank is needed for them to have room to swim around.
In any case, if you aren't used to caring for an aquarium, I would suggest starting with a simpler fish with not as many requirements.
I wish petstores wouldn't sell fish that grow big as many people don't realize their capability of growing so big. Fish like arrowana, pacu, and cichlids are commonly sold, but they can easily grow too big for most people's home aquariums. And in all reality, the feed you buy in the stores isn't going to be nutritionally complete. Many fish do best on a more varied diet.
