Ducks and geese are great birds to feed from your hand (swans can be more aggressive).
Feed them grain, bird seed, oats, rice, peas or sweetcorn. Avoid bread because this isn’t good for them.
If you’re nervous, try putting seed on the ground and gradually getting closer and closer to the birds as they’re feeding. If you’re more confident, try feeding them from your hand. Hold the food out with your palm flat. It might tickle at first but you’ll soon get used to it.