Packing your backpack can seem like daunting task, however, most people over pack and end up giving items away that they really did not need to bring.
Consider the countries you will be visiting and the weather average during the period of your stay. Climate change can be a tricky one, as you require enough clothes to survive a tropical hot summers day, but a warm hoodie in the evening as temperature drop dramatically.
Another consideration is the weight of your backpack, although your airline has given you 30kilograms to fly out to your destination, this is unrealistic. You must remember will be lugging your backpack around for your entire backpacking adventure. A good weight goal is around 10-15 kilograms.
Make sure your backpack has a waterproof cover, as mentioned above the weather can change very quickly from a lovely hot day to a thunderstorm with rainfall that will drench your backpack in a minute.