Fruit flies typically lay their eggs near fermenting food or any other moist or organic food or materials.
You should always put food away after eating it and never leave fruits or anything that the fruit flies might like on the counter and if you do leave it on the counter at least cover the fruits and other foods up so that the fruit flies cannot get to it.
You can get rid of the fruit flies by using apple cider vinegar, fruit and a jar.
Take a jar and put a little apple cider vinegar into the jar and then place part of a banana or other fruit in the jar and then put some plastic wrap over the jar and secure it with a rubber band.
Next poke a few tiny holes in the plastic wrap that will allow the fruit flies to get into the jar but not out.
The banana or other fruit will attract the fruit flies to the jar and then they'll drown and die in the apple cider vinegar since they can't get back out of the jar.