This can grow up to eight feet or more so will need quite a large container as it grows. All willows love moisture so how often you water depends on the weather. It will not like being left dry for very long but don't water if it doesn't need it.. If you potted in good compost it shouldn't need feeding this summer as the food in the compost should last. If it is OK in the present container next year you could feed a dilute feed perhaps monthly up until about August but no later as you don't want to encourage new growth that hasn't time to harden off before the winter.

Thank you