Where to start? I am very new to data science and machine learning. I need some advice on how to recognize long/short-term patterns/trends in a big data set (demand data), make predictions for future and make optimal decisions for future. I don't know where to start and I would appreciate any kind of help.

You can start with simple, relatively small Kaggle datasets and try to plot the relationships between the features and target variables, and try to observe any patterns there. With practice, you will improve and will be able to identify more sophisticated patterns/trends in real datasets, and make predictions.
