Most travelers already have a general idea about where they would like to go based on their personal interests. They may even have taken a course at university, seen a program on television or read a book that highlighted an interesting country or city in the world.
Thinking about where to go is, however, a very important part of planning a trip for several reasons. There are many places in the world today that are not safe enough to warrant backpacking there. The risks are simply too great. Different parts of the world are more or less safe depending on your race, nationality or gender. As this article is written, there are several places that come to mind immediately. But as you read this, these places may have become safe once again while other places have developed their own volatile issues. It is best to consult an up-to-date resource, like Fielding's Dangerfinder or the Travel Warnings page.
When deciding on where to travel to, consider the popularity of the destination. Are you looking to get away to a foreign country to become immersed in their culture and travel in relative peace, or is your desire to hang out with literally thousands of other tourists from the same country as yours? I can't help but think of Europe in the summer when talking about extremely busy travel destinations.
After refining your travel destination to one or two regions, consider the cities or parts of the region you will visit. Where will you begin your trip? How do you plan on getting from place to place? How much time will you have to cover your intended destination? How much money will you have? In what order will you visit each place? Some of these questions are important to answer ahead of time to help plan your trip. Some of them will simply be answered along the way. But all are important to consider when planning where to go. You shouldn't over-plan your trip -- just plan enough so that you have an idea about where you would like to go and what you would like to see.
Consider flying into one city and out of another to avoid wasting time and money. For example, if you are going to Europe, you could fly into London, travel through Europe heading south, then catch a flight out of Rome without having to loop back to London for an added cost and waste of time. Remember that this strategy will be cheaper if you can use the same airline for both your incoming and outgoing flight.
If your travel itinerary includes several different countries, try to begin your trip with the country that is most familiar to you (language, customs or just similar to your own). This way, you will hopefully get off to a good start and begin to establish the many travel habits that should become instinctive after a bit of experience. Travelers arriving in an extremely unfamiliar country may be overwhelmed by the many differences in culture, etiquette, etc.