In every other fantasy story, there is a Chosen One. Xe can alone defeat the Big Bad and save the universe. Unfortunately, Chosen Ones are incredibly easy to completely bork up.
Ask yourself whether you really need a Chosen One
If you can restructure your story so that it does not involve a Chosen One, you probably should. The general concept of a Chosen One is considered by many to be a discredited trope, so if you don’t really need it, try to avoid it.
Make the future uncertain
If you’ve decided you need a Chosen One, make sure that the outcome is uncertain - ie, that there is a chance that the hero could completely fail in xir quest. Alternatively, your character’s Chosen status needn’t imply victory at all - see Harry Potter, Avatar: The Last Airbender, and Buffy the Vampire Slayer.
Do not use your character’s Chosen status as a substitute for actual motivation
Some chosen ones - particularly evil chosen ones and dark messiahs - have no real reason for doing anything other than they’ve been chosen to do it. Whether your character is good or bad, try to make sure xe has (or gets) some personal motivation for doing whatever it is xe is doing.
Do not use your character’s Chosen status as a substitute for actual skill or practice