I was born in 1992 to a German mother and American father. My dad was in the army on tour when they met and we moved to the us when I was 3 and my younger sister was 4 months. I am now 18 and I was wanting to know do I have american citizenshi

Here is an overview of what you need to know and do:
USCIS Home Page (http://www.uscis.gov/portal/site/uscis)

Did your father register your birth with the US consulate or embassy CRBA (FS-240, Consular Report of Birth Abroad)?