It is quite hard to tell what to pack because it changes from trip to trip.
So, I might just give you some hints. First, I am going to talk about the backpack. There are loads of various kinds of backpacks but since my trips are always filled with 1 day stops I have decided to buy one with a full- length zip whips. This allows you to open your main compartment, so you can see and reach everything without having to take all your stuff out.
Second bring the less that you can – every hostel, small town etc. has a washing machine/laundry.
Third try to pack cloths which are a bit older – this allows you to wash them in every laundry/washing machine without having to worry on how they will come out. What I normally do is putting all the stuff I do not use anymore in a box ready for my next trip. This even allows you to buy some nice t-shirt or hoodie etc. leaving the old stuff behind without having to worry about having enough space in your backpack. Now I probably should start telling you all the medicine you should bring with you, but the truth is that I never bring anything with me. I do almost never use medicine therefore I never bring them with me. The only thing I do bring are plaster which are always helpful if you must walk a lot and maybe you are not so use to it.
Obviously, there are a few things which I always need to bring with me- things which I can probably define my companions:
Camera, Phone, Notes on each place I am going to stop, Pen, Music, Notebook