The most accepted answer for this is that the Greek government built up budget deficits and a national debt that were too large. In addition, they built these deficits for no good reason. When the global financial crisis hit, lenders started to see Greece as a bad risk. Greece is also in economic trouble (according to many economists) because its economy is held down by excessive regulation.
The Greek government, in the eyes of many, spent unwisely in good economic times. They spent for things that would not really help their economy grow. They also did not try hard enough to collect taxes, collecting only a small portion of what is actually owed. Between these things, they incurred large deficits and debts that became a problem when the world economy crashed.
Greece also has too many rules in its economy. It is well-known, for example, for having rules that limit how many people can work as pharmacists or as truck drivers. These rules limit Greece's ability to grow. This lack of potential for growth means it will be harder for Greece to get out of its current problems.
If you are a high school student, however, the most important part of this answer is the part about excessive government spending which led to Greece having a hard time borrowing because lenders no longer trust the Greek government.