Unless you were in the US on an exempt visa (such as a Student F-1 visa), you are considered a US Resident if you lived in the US for over 6 months in 2015. Depending on all of your circumstances, you might still be a US Resident if it was less than 6 months, or if you were in the US on an exempt visa.
As a US Resident, you are required to pay taxes on your Worldwide income.
