Decide your priorities for the trip.
Do you want to lounge around on a beach for seven days? Do you want to do a year-long trip around the world? Do you want to do an adventure tour? Do you want to learn to salsa dance? Your priorities for a trip greatly narrow down the list of possible locations.
If you haven’t decided your priorities, think about some things you’ve always wanted to do or learn, you just haven’t made the time.
Decide your budget.
If you have $500 dollars to spend on a week-long trip, somewhere like Switzerland might not be your place. Figure out how much money you have for the trip, subtract airline costs for a flight to that location (plus a few hundred-dollar buffer), and do some research on an appropriate level of spending per day in that part of the world (again, give yourself a safety buffer).
Ask some friends (or the Internet) for recommendations.
Do you have friends who have travelled extensively? Ask them for some of their favorite places, as well as why they feel that way. If not, ask your friends (like in a facebook post) if they have any friends who have travelled too much. Browse facebook groups and websites like reddit r/travel and find some places that look interesting to you. After you have done some travelling, become the knowledgeable friend that people go to for advice.
Go in the right season
Again, your priorities influence the timing of your trip. Do you want to learn to snowboard? Do you love hiking 14ers? Colorado would be a great place to learn both things, but you have to go when the timing is right. If you are only able to go at a certain time of the year, make sure that you will not be in the middle of the tourist season, or at a time when your preferred activity is unavailable. Typically, the best time to go is near the beginning or end of tourist season. The local guide places, hostels, and hotels will be open for business, but not packed to the brim.
Go where your friends are.
Do you have any friends currently living abroad? Maybe a Returned Peace Corps Volunteer who stayed after their service, or a friend who taught English abroad? Not only would it be good to catch up with them, but as they are locals, they would be able to give you advice on the best places to see or go in a country. I spent two years living in Madagascar, and could write a dissertation about the best places to see, places that aren’t worth it, hotel recommendations, etc.
Go somewhere you know nothing about and learn something new!
No matter where you go, you will have new experiences. Some of my best travel experiences have been places I was not necessarily excited about, and vice-versa. You will also find out more about what you like and don’t like in a travel destination, which will make the process of deciding where to go on future trips much easier!
Just make sure the place is remotely safe, first.