To add onto whats been said above, about storing your backpacks, many airports/train stations/bus stations have luggage lockers where you can leave stuff. Also most people are quite obliging once you make friends with them. For example I left a witch doctors stick I bought in Peru in a hostel in Lima for 3 weeks while I went off to Ecuador. No charge, no hassle nothing. I've left my backpack at a travel agency in La Paz, Bolivia while heading off to the salt flats for 4 days. Not a problem.
When you are staying in a hostel dorm, most people just dump their backpacks anywhere on the floor where they can find space. Its not really a big deal. Hostels have lockers, but you typically have to bring your own locks, and they are usually only small enough for valuables - passport, camera, any electronic gear, ipod etc.... Most places however, will store your luggage for the day for free, for example if you are catching a night bus/train/flight but have to check out in the morning due to hostel rules.
The trick, or atleast my recommendation is to keep all your valuables (passport, camera, ipod, laptop etc) in a day sack that you carry around with you, and leave your big backpack in the hostel with clothes etc....Ive done that for months without a problem, no one is really interested in stealing your dirty underwear.
When travelling in slightly more dangerous countries, like Venezuela, we used to chain up our backpacks to something in the room that isnt readily moveable like a bed. Or we would chain two backpacks to two chairs and a table, so anyone carrying all of those out would look rather stupid.
In a worst case scenario, you would lose clothes, shampoo etc, medicines, and that kind of stuff, which is all easily replaceable. Thats pretty much all I keep in my big backpack.
You can always buy stuff along the way anyway, so my advice is to take as little as you can, when you are trudging through heat or snow trying to find a hostel you will be glad to not have a heavy pack. Weigh your pack before you leave, unless you are really strong I would say anything over 13-15 kilos means you have packed too much. Take medicines from home that you are comfortable with, but things like tshirts and boxers you can always buy.
Lastly get a good guidebook, that can help you not only with where to go, what to see, where to stay but also gives you a comprehensive list of things to take. Or go to a bookstore, sit there and read one! :-)