what you have to pack depends on where you're going, when (as in what seasons) and how long for. because i have no idea what kind of trip you'll be doing i'll try to keep this as general as possible.
how much do i pack:
if you're going on a trip longer than say a week or two, you're gonna have to do some laundry. if you stay at one place for a while than you can just do it by hand and dry it by hanging it up somewhere, but if you need to do it quickly, you'll have to use a machine and most likely a dryer. in this case, make sure you don't over pack so everything will fit in one machine and don't bring any stuff you're emotionally attached to or want to keep safe, because you'll put colors and whites, delicates and all fabrics together and since you'll be using different machines you're used to back home, prepare for some damage and maybe shrinkage (but it all doesn't have to be this bad, i've never had any bad experiences, but you never know what might happen right). so make sure you use low temperatures with both washing and drying. if you put all your stuff in one machine you'll save some money too (even though it's only a few dollars, it's still worth it, would you rather spend it on laundry or on a beer, think about it!). make sure the type of clothes you bring should last you for as long as your trip lasts. i mean, if you don't mind buying new clothes all the time than bring whatever you like, but if you're on a real budget, keep in mind that you're gonna have to wear the same stuff over and over again. also don't get too attached to your socks, for some reason some always disappear...
make sure you don't exceed 20kg, just for travel reasons. first of all most airlines (i actually believe all of them) don't allow more than 20kg per person to be checked in and secondly, remember that you're gonna have to carry all your crap as well. oh and also, you'll be buying souvenirs so have some space left for extra stuff.
suitcase or backpack?
personally i'd say backpack. it's easier to carry and you'll have two hands left for other stuff (to hold a map, pay for your room, give someone a hand, have a drink, smoke a ciggie, you get the point). most backpacks are built for travelers anyway, so they always have handy gadgets and thingies to make your life a bit easier (for example some backpacks have wheels now). here you can find some good brands, compare prices and find a backpack that matches your needs. my backpack was 55L which is absolutely perfect and i've been traveling for about a year and a half now. before investing in a backpack, think what you're using it for and decide how big it should be (keeping in mind that you won't exceed 20kg). backpacks are really strong and will last for a long time, so you will be able to use it for more than just one trip and i guess if you change your mind later on, you can always sell it to someone. and if you don't want to invest in a new backpack just yet, check for second hand ones online. you can always buy a new one later in life.
a daypack is very handy as well. i use my camera bag as daypack, but if you're not bringing a camera than just bring a daypack with enough space for luggage transfer in case you're flying with one of those domestic middle of nowhere airlines who don't allow more than 15kg. if not than just take one with you with enough pockets to put passport and important stuff in and isn't too annoying to carry around all day.
stuff you should always pack:
* first aid kit (plus extras depending on your destination, for example snake venom) * pocketknife (always comes in handy)
* flashlight (preferably one without batteries)
* photocopies of your passport and other important documents, put them in different places in your bag(s)
* plenty of any medication you're using
* deck of cards (for social situations)
* travel guide/ language book (if necessary)
* traveling sewing kit
* stuff you need that you'll run out of and is not sold in the country you're going to
handy tips no one ever tells you:
* roll roll roll. that's the best way to pack a backpack. roll your clothes and that way you can stuff them everywhere in between and your backpack won't be "full" as easily
* put some books or other hard stuff at the bottom of your backpack so it won't keep falling over when you put it down somewhere.
* make sure you put stuff you don't want to get wet in a waterproof plastic bag (most backpacks already have these)
* attach some sort of scarf or other piece of fabric on the outside of your bag, so you'll be able to recognize it anywhere (for example when claiming your luggage)
* when flying, wear your "heavy" clothes to make sure you won't exceed 20kg*
