Not sure what species it ism but it's definatly a salt-water fish (as is Nemo), which is not recommended for first-time fish keepers.
They're usually very expensive.
Do a small water change (about 10%) for 3-4 days, (Making sure you put tap-safe in of course)
After this, try putting some aquarium salt in (Available at pet stores)
I find salt to work better of illness' and diseases than most medicines you can buy, also, it prevents stress, which can cause...