Pack it down a bit with hands or a shovel. Then you are ready to begin. Some professional sculptors add glue or soda to water to make castles a little tougher but natural salts normally present in sand and in water will suffice to give castles a crusty firmness when dry. Most professional castle builders will go to great lengths to firmly pack their sand. This also make's the castle stronger.
Tools are critical to castle success.
I use popsicle sticks and plastic spatulas ranging in size from about half the width of a credit card to about twice that width. The spatulas seem to work best if they have a slight bend in the middle but flat pieces of plastic like credit cards can also be used with great success. Masonry trowels are also very useful but they have slightly less safety because of their sharp steel tip. Once you''ve got a nice packed wet pile of sand. And at least some tool.