The first thing you should do is eat and drink slowly. If it was a hot race, you will have a strong desire to chug a gallon of water and avoid food. If the race was cold, you might feel the opposite. Either way, you need to drink water, and do it slowly. For food, take in high glycemic, easily digestible foods. Aim for a 4:1 carb to protein ratio and avoid fats, as they slow the digestion of the other macros.