The contribution of various sectors to Nigeria's Gross Domestic Product (GDP) was investigated in this report, as well as the creation of a model for forecasting Nigeria's GDP over a 33-year period (1981-2013). Regression analysis and time series analysis were used to analyze data from the Central Bank of Nigeria. The regression results show a positive relationship between the three sectors: Agricultural, Industrial, and Service, with only the Industrial and Service sectors contributing significantly with coefficients of 0.286 and 0.631, respectively, while the contribution of the Agricultural sector is not significant with a coefficient of -0.039, implying that the service sector contributes the most. The agricultural sector makes a negligible contribution. A time domain model (fundamental approach) based on the Box Jenkins approach was used to forecast Gross Domestic Product for the period 1981 to 2013 using the ARIMA model in a developing country like Nigeria. The outcome shows an upward trend, and the series' first difference was stationary, indicating that the series was I (1). ARIMA was found to be the best model for explaining the series using expert modelers (1, 1, 0). The diagnosis on such a model was verified, the error was white noise, there was no serial correlation, and a 10-year forecast was made, indicating that GDP would continue to rise over the forecasted time period.