The first thing to check when a fish is not well is the levels of ammonia, nitrite and nitrate in your tank water. You really need your own test kit, but for now can you take a sample of your tank water to an LFS and ask them to test it - make sure they give you numbers.
How often do you do water changes and how much do you change?
Do you clean the gravel at every water change?
Not enough water changes can allow things to build up in the water, and not cleaning the gravel allows debris to build up in there. These can make fish sick.
Dropsy isn't an illness itself, it's a symptom of several other diseases. To treat it, we need to know what the underlying disease is. It could be a bacterial, viral or protozoan infection which need different treatments.
One thing you could try is an epsom salt bath as epsom salts draw out fluid from the body. Epsom salts can be bought from pharmacies but they must be pure with no perfume or colouring added.
Take some water from the tank into a small tub. Add epsom salt at the rate of 1 teaspoon per gallon and dissolve it thoroughly. Place the fish in this for up to 30 minutes but remove the fish earlier if it becomes distressed.
However it would be unfair of me not to warn you. Dropsy occurs when the kidneys are damaged by the underlying disease and while there are cases where a fish has recovered, this is not common as the kidney failure has usually gone too far by the time dropsy develops. Please be prepared for this.