Before you go on your trip, check out what the weather will be like at your destination. You can easily check this online.
This will determine what you need to bring and what you can leave at home. Find out if it's the rainy season in the place you'll be visiting, or if it will be cold, etc. There's no point in packing a sundress when it will be overcast and rainy! If you check the weather first, you won't need to go and buy a new raincoat if you wind up someplace rainy!