When travelling I would always take two bags. My main backpack which would have the majority of things in and would go in hold luggage on planes and buses. You would probably be best with a backpack between 60-100 litres in size, depending on how much stuff you want to take. I had an 80 litre rucksack. I would then have my hand luggage, which would be a rucksack. I would often wear this on my front when walking to and from bus stations or to hostels or anywhere with all my luggage. I would use my hand luggage to keep all the most important items in and keep it with me at all times. So use it as hand luggage on trains, buses and planes. Then when getting to a hostel, I would empty this bag out and put all the important stuff such as passports, documents, money in the safety box which most hostels have in the rooms, failing that just in my pillow case. I would then use the rucksack for days out and trips to put food, water, my camera and any other stuff I would need in.
I also had a document folder, which I kept all my important documents in, including some spare money and my passport. I will also provide a list of stuff you might want to put in your document folder again you might not want to use it all. Using a document folder keeps everything safe and organised, so you know exactly where everything is should you need anything. It saves you panicking and having to search through all your bags, I just keep this in my rucksack.
Hand Luggage (Rucksack of some sort): Now this packing list for your hand luggage may look very long, but a lot of it is very small things that you can group together into different bags, such as electronics and wash stuff. If you're not flying long haul wash stuff and things like toothbrushes you won't need, but if it's an overnight flight, trust me, you'll be grateful you packed them. Also remember in your hand luggage, you are allowed no liquids over 200ml, so buy the travel shower gels and deodorants and obviously no sharp or explosive items.
Main Backpack: Again this is the packing list I used and it had everything I needed on it, you may want to add your own stuff or take stuff off depending on where you are going, this is just your average list. If you're going somewhere cold, obviously take a coat. Again adjust depending on how long you're going for. Also females will want to add a few extra things that i haven't mentioned.
Documents: These are the main documents you should take with you, just in case you lose your passport, or your phone dies, or you need something for getting through customs in another country. Better to be safe and have it than sorry and go home.