First things first- you need to start your climb with a bag that feels comfortable on your back. You bag will carry the rest of the essentials in it, so you need to make sure you get something sturdy as well as comfortable. A rucksack shouldn’t make your neck and shoulders hurt- look out for soft padding around the straps. Ideally, a 45-55 litre backpack should suffice. Also look out for one that comes with a detachable daypack.
Where to buy: Bragpacker (you can also rent it from them)