There's no foolproof, one size fits all, right way to choose where to travel. There are several motivators that I think make the choice more obvious, like ancestry, travel aspirations, the time of year you can travel and for how long. I think foremost of these are places you aspire to travel, and ancestry, if you want to visit a country you or your family has a connection to.
I think if you're not sure where to travel, however, there are some ways to plan your trip that, in my opinion, give you a more fulfilling travel experience.
Travel Goal/ Heritage
Starting with the most obvious, if you have somewhere in mind, your next step is laying out a reality where you make this happen! I cannot emphasize enough the importance of research! It is so easy nowadays to type in keywords and find hundreds of resources for your questions about places or tours. With a place in mind, the next best factor you want to use in your search is when you would like to go. Depending on your reasons for this goal, or if your family has heritage in a place, or if you simply love the history of a certain region, having a time of year to hone in on can help you plan a route and activities. Focus on experiences you'd like to have instead of locations you'd like to cross off, this way you have plans at your destinations, and you can use those as great anchors for asking your tour providers or locals about other things to do or see. This is where forums come in so handy; you can read from experienced travelers what did and didn't work for them.
So, if you don't have a place in mind, start by narrowing it down by the time of year you're traveling. I think it's more important than many travelers do to choose a time of year that suits you as far as weather and tourism peaks are concerned. For example, in planning a trip for my birthday, I noted the best time of year for nice weather and not much crowding in Thailand would be November, and since I can be flexible with my travel schedule and prefer low season for prices and crowds, I decided that would be the perfect place to go. It's also ideal for the length of time I'd like to travel, (the whole month) since I'll have ample time to explore many islands.
Many people can only travel during the summer or holidays, and while this limits you in some ways, it's also an opportunity to explore! Looking for off season deals is my favorite way to discover new places.
Events or Festivals
Events already have a time and place, so your best planning around an event is going to be around where to stay and what to see in the area. For this, I love guide books or magazines about specific places. Some examples of this kind of planning would be like going to the Voodoo festival in New Orleans and using the time there to explore the city. The options for lodging nowadays are endless. From sharing someone's couch on couchsurf.com to renting someone's entire luxury condo on AirBnB, you can tailor every part of your experience. Lowkey hostels and guesthouses open you up to a lot of options in international travel, and I also recommend this as a great way to have a more local experience. In 2011 in Sri Lanka, we mostly stayed in guesthouses with locals. Often, these hosts cooked us dinner and directed us to tours and local activities.
This may be too freeform for many travelers, as some like to have their entire itinerary locked in before leaving. These are the travelers whom I usually recommend cruises to, since those have your lodging and itinerary mostly nailed down and let you explore several destinations on one trip. It is entirely possible to book all your activities and stops before you leave, it really just does depend on preference.
At first you may feel lost, but don't shy away from this feeling! The magic of traveling is that, unlike your everyday life, you can form the entire experience into whatever you want it to be!