Decide on a budget
How much money are you willing to spend? A thousand? Two thousand? Five hundred? This will help you make the rest of your decisions.
Decide on a time frame
This can and should be just a rough idea. How many days can you take off work? What month or time of year will you be available? What destinations fit your budget? If you can, avoid nailing down exact dates until a little later. This will save you a ton of money.
Decide on a place (or two)
Again, the more general you can be, the more money you will save. Is there one set destination you want to visit, or do you have an ever growing list of places you want to see? What places on that list are top priority? What places on that list fit the time frame or time of year?
Look for flights
You want to give yourself time to do this. Start looking in advance. Look at different cities on your list of possible places. Find out what the average cost of tickets is, and then look for anything below that. If you give yourself around a month to search for flights before you need to start booking, and remain open to multiple destinations and dates, it is highly likely that you will stumble upon a deal. When you find it, book it.
Start your research
What are the top things you want to do and see while you are there? Will you need to travel around or be in one place? Where will you want to stay? Read and read and read and look at pictures and read. The internet will be your best friend.
Once you know the places you will want to stay, look for and book your accommodations. The further out you do this, the wider selection you will have. If you are like me and are likely to change plans, use Booking.com, as most cancellations are free.
Will you need tickets to Machu Picchu? Are you taking the train from London to Paris? Are you taking a boat ride along the coast? Research and book all of these beforehand. This will usually save you money, ensure you get the dates you want, and save you the hassle of doing it once there.
Do more research
Now you can be more specific to the places you are going. What language do they speak (and what can you pick up), how do they tip, what customs should you be aware of, what food do you need to make sure you try? Learn about the history, the culture, and the people. This will make the trip all the more meaningful.
Make a list and check it thrice. Pack light but smart. Chances are you came across packing lists for your destination in your research, so now is the time to bust them out. Check the weather and pack accordingly. Remember to consider your means of travel when packing. Will you have to lug your suitcase around the subway? Is it just going straight from the airport to your rental car? This will help you decide how much you can bring and what type of bag it should go in.
Print and double check all the things
I know you have email confirmations galore, but your phone will die. Print that crap out. Email yourself copies of important documents (Passport) in case of emergencies, and send your itinerary to someone who cares (mom). Double check any last minute things, and then proceed to forget at least one important thing because you are human. It will be fine and you will have fun.