Better to have data of previous two decades and then you can use statistics to predict by extrapolation.

I have data of last 6 years.

If the data are annual little or nothing can be achieved with a sample of size 6. If the data are monthly you have 72 data. A time series model can be fitted to the set of data and extrapolation done for forecasts.

Thanks Mr. Ette Etuk, i have data of Can you please suggest a reading for time series model and extrapolation for forecast.