How to predict future sales based upon last year sales data?
You can not predict sales for the whole next year by training your model with only one year data. 

But still one year data is better than having nothing. And you can analyze one year data to find trends and patterns. On top of those trends and patterns, you can apply domain knowledge and make some very some useful strategic decisions. 

Problem you are trying to solve is called Time Series Forecasting as temporal order of input matters while training. 

Here are reference points you can your start with : 
1) https://machinelearningmastery.com/how-to-get-started-with-deep-learning-for-time-series-forecasting-7-day-mini-course/ 
2) https://www.analyticsvidhya.com/blog/2018/02/time-series-forecasting-methods/ 
3) https://towardsdatascience.com/an-end-to-end-project-on-time-series-analysis-and-forecasting-with-python-4835e6bf050b