banana can be buried in the soil directly, composted, or you can make a tea from it by fermenting it in water, or you can take it further with JADAM as animistic suggests.
if you can, start a compost bin or a worm bin. worm bins are a better option, though for some reason i haven't tried it myself yet.
i put all food scraps in my compost bin and make various fertilizers from coffee grounds, potato peels, bones, egg shell, my own pee (one i am particularly fond of), wood ash, weeds from around the garden and whatever else i can get. you can often get horse manure and other farm waste for free if you look around. i'm sure i forgot lots of things. most things can be fed to plants. even meat can be composted despite common belief, though i don't waste food. i have not got around to humanure yet, but i will be doing it in the future.
i also regularly top up my compost piles with whatever i can get from a local woods. i visit there regularly anyway so when i go i fill a carrier bag and add it to my compost. you should be careful not to disturb local wildlife if doing this though, like bugs nesting in leaf piles etc.