Hi aim, welcome to the board. We usually just use this section for newbie introductions - you might want to post in beginner's questions or in emergencies with questions like this in the future - you'll tget lots more responses!
Anyways, the condition you are describing is commonly referred to as popeye. Popeye is a bacterial infection, and can be treated with many different antibiotics. I use tetracycline for popeye, but whatever is available to you would probably be okay. Look on the label, it should specify that the antibiotic treats popeye.
One more note - many diseases are caused by poor or fluctuating water quality. One of the best things you can do for your fish as a preventative measure is to get a test kit to moniter the health of your aquarium. This is the easiest way to undetrstand what is happening in your tank and to prevent future disease outbreak.