To start an app business, begin with idea generation and validation through market research and user feedback. Define your target audience and develop a business plan that outlines your app's value proposition, revenue model, and marketing strategy. Assemble a team of professionals, set up the legal and financial aspects, and develop a minimum viable product (MVP) to test feasibility. After completing development, launch your app on desired platforms, and execute marketing strategies to drive awareness and engagement. Continuously monitor performance, gather feedback, and optimize your app for growth and scaling.