You need a passport to go to Nicaragua. If you have a current passport in your current legal name, make sure it does not expire before August, 2013, or else it must be renewed. I recommend that you apply for your passport at least 3 months in advance of the trip (by May, 2013).
To obtain a passport for the first time, you must apply in person (most post offices and some public libraries accept passport applications), and there is a fee.
For information on how to obtain a new passport, or renew an existing passport, go to:
NOTE: You do not need an Adult Passport Card.