Army Inquiry

Does anyone know if it is possible to locate information about a person who was in the Service and if so in simple language how to locate that persons records?
I have a great uncle who was in the Army in the early 1940's.
I don't know if he was in the war or not.
He was born in New York.
I would really like to find out more about his time in the
service but I can't figure out if or how to get the records or what information I will need to obtain them.
Can anyone help?
Thanks,
Pat