first, do some research on the top vacation destinations in the world ( if you can afford to travel the world) or within your continent. Planning your vacation should involve a lot of research in terms of what to expect and the costs it will entail.
Word of mouth ( experiences that your family or friends tell you about) also is one source of choosing your destination. Whatever interests you should point you to where you are going. For example, if you the beach, then definitely, when you plan, you should look out for countries or places where you can find them. List your choices and based on costs, or interesting sights, etc, you can now trim down your choices until you get to you desired destination.