Water has been to many cultures symbolic of the source of life. Sacred wells or bubbling springs of water, to pagans, were often seen as having a spirit or deity protecting them. It was believed that if you were to make a wish by one of these wells, it would be granted, if you left an offering to the protective deity of the water.
This is where the custom of the wishing well comes from.
Use a coin as your offering, as you drop it in the waters below, imagine your wish and ask for it to be granted. Trust that your wish will come true, leave your wish at the well... and don't tell anyone what it was that you wished for.