Go to the fish store with a list and a calculator. You need at its most basic: 1 tank, 1 cover, 1 filter, 1 bottle of conditioner, 1 bottle of food.
Optionally you might want a light, an air pump (with tubing and air stones), a background (which is easy to make yourself) and any decoration you like, gravel for the bottom, A stand if you have nothing sturdy to put it on.
You'll probably find the kits cheaper, but they don't come with that much and the filters tend to suck. If you have little money then keep it simple and just focus on the first items I mentioned - that's all you need to keep a goldfish.
