Hi Chikon,
You will need to carry your backpack around for long periods of time. Therefore it must contain everything that you will need while you are away that can’t be purchased locally.
The important part is backpack weight as lightly as possible. Because your backpack will need to be carried with you everywhere and having a large bulky pack could tire you around the trip. You should get atleast 2/3 T-shirts, At least one long-sleeved shirt or polo shirt, 2 pairs of trousers, good sleeping bag is essential, Water bottles, towels, Plastic bowl with knife, fork and spoon, Travel washing line.