Thinking of going backpacking? Here you will find everything you need to know on everything you need to bring, especially to keep your load light. I will list and discuss the bare basics, though anywhere you go you will need to pack slightly different, depending on the country, climate, and what you are travelling there for. It is really hard to go backpacking for the first time, as you tend to over-pack and waste space, and/or forget to pack essential items.
Backpack - Obviously, you will need a backpack. Depending on how comfortable you are with travelling, this will really factor in to how big your bag will be. Smaller bags tend to not stick out, will blend in, and make you less of a tourist and less vulnerable to pit pocketers or being robbed. Again, depending on your destination, the things you pack will vary. A good medium sized pack should do the trick. You don't need a gigantic hiking bag as this will be very annoying to carry around, and will make you more prone to over-packing things and clothes you don't need. Though, the size of your backpack is really up to you and how comfortable you will want to be.
Clothing - Most of us have all gone on vacation and find you packed way too many clothes and didn't even use half of them -- at least in my case I have. It makes your suitcase cramped, and gives you less room for things you buy from wherever you go.
Backpack travelling is different. The bare minimum, essential clothing you should take backpacking with you are:
2 plain loose cotton t-shirts
2 plain tank tops/wife beaters
1 nice shirt (even just a collared shirt or polo, for nicer occasions/dinner)
1 light jacket or zip up sweater
2 pairs of pants (leggings, trousers, khakis, active wear pants)
1 pair of shorts (obviously optional depending on destination's climate)
7-10 pairs of underwear (girls underwear is a lot smaller, thinner, and easier to pack, guys may take less)
2 pairs of socks (ankle socks are best, make sure to bring two just in case one pair gets wet)
1 bra & 1 sports bra (obviously girls only)
This is just what I recommend, but everyone may pack clothing different. Remember, you can always buy clothing wherever you are usually, so you do not need to bring all the clothes from home you will be wearing. You will feel a lot better only bringing the essentials.
I also recommend not bringing jeans -- they are heavy, take a long time to dry, and can get hot to wear. Also, clothing like UnderArmour are great to wear if you are going to be active, since they are sweat wicking, lightweight, comfortable, and keep you cool in hotter climates and warm in cooler climates.
Footwear - You don't need a lot of shoes, at the most, 2 pairs. In hot-mild climates, all you need is a pair of light runners and light sandals or flip flops. You could even bring a pair of thin shoes like TOMS, as they dry quickly, comfortable, and lightweight.
Steer clear of hiking boots. They are bulky, heavy, and take up way too much room. Unless you absolutely need to have them, like you are travelling for the purpose of going hiking, you won't need them. Again, remember you can buy shoes wherever you go, so if you need an extra pair, it won't be a problem.
Umbrella - It will probably rain at some point, so brining a small compact umbrella is a good essential and won't take up any room at all.
Hat - Either a baseball hat or bandanna you should bring, especially going to a hot country to protect your head from the sun.
Water Bottle - Not a big bulky 2L water bottle, but a flexible water bottle that can go flat and roll up when not being used. You can fill them up anywhere and it is always good to have water with you.
Swiss Army Knife - You never know. You probably won't be able to get away with this at the airport, but once you arrive at your destination, any convenience shop will carry them and they are just a good thing to have handy.
First Aid & Medicine Kit - Find a small, waterproof bag to act as your first aid kit. You should at least bring a travel size Advil or Tylenol, travel size Pepto Bismol or Imodium, a few bandaids, a small pack of tissues, cough drops, hand sanitizer, and lip chap. Again, you can buy this stuff and more when you are there most likely, so just bring what you think you will only absolutely need.
Hygiene - All you will need is shampoo, conditioner, face/body wash, toothbrush/paste, deodorant, and a razor. Try to find get travel size shampoos, even a 2 in 1 shampoo and conditioner is better since it will cut down on space. You can also find a body wash and face wash that will work for both, as well as a razor for your face (and body). Shaving cream is optional and a personal preference, though I can shave just with water or just with body wash and will both work. All these types of things can buy replaced and purchased there though, so you don't need to pack everything before you go.
Female Hygiene - If you are going for a longer trip and/or know you will be getting your period when you are gone, a small box of compact tampons should be fine.
Sunblock/Sunscreen - pretty self explanatory.
Other Essentials You May Consider:
compact digital camera
secure zipper money & passport pouch
plastic fork and knife
small bag of makeup
lightweight thin blanket or sheet (for sanitary reasons)