To build a weight-loss app like Lose It!, start by outlining key features such as calorie tracking, exercise logging, and a comprehensive food database. Conduct market research to understand user needs and preferences. Develop a user-friendly interface with intuitive navigation and an appealing design. Incorporate functionalities like barcode scanning for food items, personalized meal and exercise plans, and progress tracking charts. Implement strong data security measures to protect user information. Utilize APIs for database integration and consider collaborations with fitness and health service providers. Thoroughly test the app to fix bugs and enhance user experience, then launch it on multiple platforms with a solid marketing strategy to attract users.