I'm guessing you've either seen Bala (silver) sharks or red-tailed black sharks as these are the most popular and widely available in the hobby.
Bala sharks need at least a 75 gal, preferably bigger tank due to the size they reach so don't even consider them unless you have this in mind. They are schooling fish and should be kept in at least groups of 3. They are very peaceful fish.
RTBS need a minimum of 25 US gallons, again preferably more. They should be kept on their own as they are highly agressive towards their own species and any other shark species or even similar shaped fish.
They are both great species but do your research if you are considering getting either of them.
Hope this helps