My goldfish keeps floating upside down!

Like you said, it's likely swim bladder disease.

 

Best thing to do when your goldfish is swimming abnormally, is not to feed them for a couple of days, since most problems with the swim bladder has to do with problems in the digestive tract Now, after these two or three days, you should feed them with peas, natural frozen peas that you thaw and pop out of their shells before feeding. It could be good if you'd test your water, because high nitrates are usually a pretty big cause of swim bladder problems as well.

And about their diet, you'd better buy some pellets and flakes meant for goldfish! You could still feed them the current food, but just from time to time to add variety Goldfish have very different nutritional needs than tropical fish, and that's why there's special goldfish food available in the first place.
You should supplement the staple diet of pellets/flakes with vegetables, lots of vegetables! For example peas, cucumber, lettuce, zucchini or boiled carrots. Bear in mind that none of these should be spiced in any way. The cucumber should be peeled and sliced and the lettuce can be attached in a clip with a suction cup to the glass, or "planted" in the gravel. You should leave the vegetables in the tank for at most 24 hours before you remove the remains. I'm not saying that you should buy vegetables and cook them exclusively for your fish, but you can give them a piece whenever you're cooking or otherwisely having them around
Fruit are also well received by goldfish! Banana, orange, water melon, strawberries... but you should feed this only on occasion and do a major water change afterwards.

Along all this, you should still feed them with some live foods, frozen, freeze-dried or packed in gel You can choose from a bunch of different kinds, but Bloodworms and tubifex are the most common, and they would be perfectly enough! Aside from that, you could give them brine shrimp, krill, artemia etc etc. Go check in your pet shop what they've got!