Besides the backpacking basics of clothes and toiletries, you will need a few extra things to help your trip run smoothly.
Buy a nice backpack. Your back will thank you. This is one big investment you will not regret making. Traveling around the world, I seem to always run into red Osprey bags. I loved mine backpacking through Europe for three months, and I could walk straight afterwards, so I take that as a good sign. The ones I have seen and used have chest and hip straps and are shaped to your body with a hard lining allowing for maximum ergonomic support.
Have a lock. You never know when a lock can come in handy, and it should be top on your list of items to pack in your bag. Your backpack is your home on the go, and you want to protect its precious contents. Most hostels will have bins for locking items, but not the locks. I prefer the dial padlocks because I do not have to worry about losing a key.
Pack an international adapter with surge protector. There is nothing worse than bringing a bunch of electronics and not being able to use any of them, or worse yet, destroying them. If you have any intentions of visiting out of the way locations or third world countries, do not plug in your electronics without a surge protector. Electricity commonly goes out, and when it does, it fries your items that are plugged in if they are not protected and running. My friends and I have lost many a computer and cell phone this way. Do not make the same expensive mistake. Not every country has the same design of outlets, so you will need different shapes and sizes. Tripshell offers all in one for an affordable price on Amazon.
Take an unlocked phone: Phones today can be your computer, phone, music, and camera. Just make sure yours is unlocked and can have its SIM chip, a small circuit inside your phone with all the subscriber information, changed. Cell phone providers lock your phone so that you are unable to use SIM chips from other providers and to prevent people from stealing your phone and being able to use it. Most countries outside of the USA have pay-as-you-go SIM chips and without an unlocked phone, you cannot take advantage of this easy and affordable way to get service in each country. A chip will run from $1-$10 and after that you pay as you go, adding money and selecting packages for talk, text, and internet time. Most second hand cell phone stores or general cell phone stores can unlock your phone. Phone providers can unlock the phone but many have rules that prevent you from unlocking your phone, especially with i-Phones.
Bring a water bottle: Most countries use purified water for drinking. While some have perfectly good drinking tap water, others do not. Either way, it is a good idea to keep your eyes out for free purified water containers and fill up when you can.
Bring a book: So often people buy books only to have them gather dust on their bookshelf. Pick one of those books. Backpacking, you should have plenty of time to finally delve into the imagination. My problem is always that I want to bring more than one. You really do not need to though as books exchange baskets are commonly found in hostels. When your done with your first, just trade it in for a new one.
Keep important items safe: Make copies of all your forms of identification. Store your originals and spending money on your person and copies and extra money in your backpack. Having copies will get you a passport much faster from the embassy if you loose your IDs and will keep you out of trouble.
Bring a quick dry towel: There is nothing worse than getting to a hostel and finding out they do not have towels or having to use a soggy moldy towel that smells bad. A quick dry towel is the answer to your prayers. It lives up to its name and will be barely noticeable in your bag it is so compact. REI has a variety of colors and styles to choose from.