Will anyone point me to a sales forecasting tutorial?

Hello,

I need to develop a multi-item/multi-location sales forecasting model to replace a current 20 year old model in production. I’m seeing a lot of tutorials on stock price prediction / single item foreacasting prediction, which is great, but my model is taking all my items and locations and assuming their all the same.

Is there something out there that can help guide me on how to do this?