Help me select what to wear for a beach party with my friends and what clothes to bring in my backpack so I can change for dinner. The weather will be hot during the day and cool at night. 

Here are my options: A winter coat, a bathing suit, sandals, a short sleeve button down shirt, a hat, sunglasses, underwear, socks, sneakers, a rain coat, a tuxedo, sweatpants, jeans, a tank top, a pair of khakis

Since you will be at the beach and it will be hot you will want a bathing suit, sandals, a hat, sunglasses and a tank top. 

In you backpack you should bring a short sleeve button down shirt, underwear, socks, sneakers and jeans to change into for dinner.