There are a number of things you will want to have sorted and done before you go travelling, this is not a packing list on what to pack, these are things you should have sorted before you head for the airport.
Bank Account- Let your bank know you're are going away. Sometimes if you go abroad and the bank are unaware, they might stop you from being able to withdraw money. Which is for obvious reasons disastrous. So let them know your plans.
Make sure you have a valid passport and it has at least 6 months left on it from when you are planning to leave.
You will need a place booked/have somewhere to stay for the first few days when you arrive, and make sure you have the address of the place you are staying. This will save you a lot of stress looking for somewhere when you arrive.
Make sure you know how to get from the Airport to where you are staying, are you getting a shuttle or public transport. Do some research, find bus numbers and times and how long it takes. Again this will save you a lot of stress when you get to the airport, because you'll know exactly what you're doing and where to go.
Take about $100/£50 cash with you minimum for when you land. You never know what you might need it for and it's always good to have some local currency, just in case there is a problem with your bank for example.
Obviously you will have some flights booked whether it's a one way ticket or a return.
Get some travel insurance bought, so it is valid from the day you arrive until the day you leave. Usually the cheapest way is to get worldwide insurance for a year, especially if you have a few countries to visit.
Make sure that you have a visa and the confirmation all printed off so when you pass through customs on the other side you have proof if they ask for it. You usually won't be asked as it is done electronically, but better to be safe than sorry.
Immunisations, make sure you have been to the doctors and are up to date on all the necessary jabs. Tell them where you are planning on going and make sure you're all up to date.
If you have medication, make sure you have asked your doctor about this and get all the medication you will need for the length of time you are away.
Take some spare passport photos, just in case, as you never know when you might need them.
I would also have a document folder which you should keep in your hand luggage and just keep all the important documents in it such as; passport photocopy, visa copies, passport photos, any confirmations such as accommodation, bank information and print bank related emails, travel insurance, flight tickets etc.
Also plan a few ways which you can keep in touch with family and friends such as Skype, Facebook, email, whatsapp.
Make sure you have a rough idea of where you want to go, and the things you want to see and do. Obviously give a chance for your plans to change, as you'll meet people and do things you didn't plan. But you don't want to arrive and have no idea of what you're doing or where you going.