A 5-Step Process to Forecasting Sales

Dear Dr. pROfIt: I'm a web analytics expert, but my boss recently asked me a question that has me stumped. She asked me to predict what our e-commerce sales will be in 2015. We currently generate $35 million in online sales. Should I just increase sales at the rate of inflation? Should I purchase a research report that outlines e-commerce growth over time? Honestly, I have no idea how to approach this.