I want to plan a vacation in summer. How do I know where I can go?

There are multiple steps to deciding what the best destination is. First, you need to think about how long you want to go on vacation. This will determine how much travel time will be adequate. Second, you need to think about what type of vacation you would like and when you want to go. Should it be a relaxing vacation with warm weather? Or do you want to go sightseeing in a city with mild temperature? Do you want to go hiking? Or skiing? Climate zones are very different, therefore it will depend on what kind of vacation you want to have. Third, you need to think about getting there. Do you want to be able to drive there by car or take the train? Or do you want to take a plane? Importantly, you also have to think about the budget for the overall vacation. Some destinations are more pricey than others. 
Following these steps will narrow down your list of potential vacation destinations.