It depends on the age of the animal for size of the prey and frequency. Some people feed hatchlings/young ones as often as every five days but most seem to feed once a week.
As to prey size, well...rule of thumb is to feed a prey item no bigger than the one and half times the diameter of the largest part of your snake. For example, if your ball is 1" around at it's widest point, the largest reasonable prey item would be approximately 1 1/2" in diameter. Some people do it based on weight and feed 10-15% of the body weight of the animal for their prey item.
Most people feed rats for food but mice or rats are both appropriate. Some schools of thought seem to think that the snakes develop better/more quickly in terms of growth/weight gain on rats but that's up for debate. African Soft Fur Rats are another food source but can be addictive, so unless you're absolutely certain you'll keep the snake for the duration of it's life span and will be able to provide it ASF's for it's life, you might steer clear of those.
As to a difference between corns and balls, balls have a higher humidity requirement and are a little more picky about their husbandry. I have both and my corn is pretty much bombproof, whereas the balls are a little more work to manage the husbandry of. Seems like you're doing your research and looking into things before you take the leap, so keep that up and you'll do fine. Besides, you came to BP.net and it's one of the greatest sources of info for sure, lol!
Good luck!