Obtaining US Citizenship is both incredibly easy and incredibly difficult. Allow us to demonstrate these two diametrically opposed descriptions. On the one hand, if you are born in the United States or in many U.S. territories, you obtain citizenship at birth. It's so simple that you don't have to do anything except be born. However, for those who were not born in the U.S., the process for obtaining U.S. citizenship -- a process called naturalization -- is far more difficult.
You can apply for citizenship through the naturalization process by filing an N-400 application. You have to be at least 18 years old to do this. In more complex situations, someone who is under the age of 18 can be naturalized if their parents are U.S. citizens. This often happens in the case of adoption. In such a case, form N-600 is used to potentially help the young person obtain U.S. citizenship.
Criminal history often complicates any effort by a person who wants citizenship. However, if you have successfully expunged your criminal history, it could help your cause. Still, you should be forthcoming with your history and inform U.S. Citizenship and Immigration Services when you are going through the naturalization process.
Ultimately, you may still be denied even if you think you have a legitimate case for citizenship. However, don't be discouraged. Even when you are denied, there is still hope. There is an appeals process that could earn you the result that you think you deserve: the granting of U.S. citizenship.
Source: FindLaw, "Citizenship FAQ," Accessed April 14, 2015