given time, any food will decompose to ammonia.
this is one, and I believe the main, reason that you don't want to overfeed your fish (once you get everything set up)
to cycle with fish:
Pick yourself out a "hardy" fish, maybe a platy. Your tank size will determine how many.
If you have any friends who have tanks, see if they wouldn't mind running your filter in their tank (in addition to theirs). This would "seed" your filter with good bacteria, if you can't do that, see if you can get some bacteria in a bottle (Tender Living Care, or some similar product).
Having a base of bacteria will help - but isn't a necessity.
Put your hardy fish in your tank, feed sparingly. Watch the ammonia and nitrite levels. The ammonia will spike, then start to dissipate and then the nitrite will spike and start to dissipate.
To put your fish in a little bit of ease, try to do frequent water changes. You need the ammonia to be in there to cycle, but you don't need to have TONS of it. Every water change will remove some ammonia during this process. The fish will replace it constantly.
Some people use the feeder fish for this process, but there are those that don't like to do this because of all the fish, feeders have a higher chance of having disease.
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and the last piece of advice - research research research, never hurts to know more
