Goldfish are not really that easy to breed. Best thing is to have an outdoor pond. In aquariums they are petty challenging and only few people have done it. They need to be a pretty good size too.
Goldfish can breed at one year of age, but produce better when 3 years old. They can be bred in large aquariums. You will need some spawning mops or plants. Eggs will have to be removed or they will be eaten.
^ yep. The males will start displaying breeding stars and the females will get plump. A good water change and throw in some mops and the males usually start chasing the females into the mops until they drop eggs. It can be easier to move the goldfish to a new tank in some cases and leave the eggs to hatch where they landed.