what is the most important thing to start with

The most important thing to start with depends on the context of your project or app development. Here are some key starting points from different sessions at WWDC:

3D Asset Optimization: When creating 3D assets for spatial computing, the most important thing to consider is how your content will be viewed. This includes whether it will be an immersive app, a volume in your space, or in a shared space. These choices impact performance and content optimization. Optimize your 3D assets for spatial computing.
Foundation Models Framework: For working with the Foundation Models framework, the most important part is the model itself. Starting with prompt engineering in Xcode is crucial to effectively use large language models. Meet the Foundation Models framework.
Assistive Access: When customizing your app for Assistive Access, start by identifying the core functionality of your app. Focus on one or two essential features to support, which helps reduce distractions and cognitive load. Customize your app for Assistive Access.
Design Foundations: In app design, start with a solid structure that shapes navigation, highlights important elements, and enhances the user experience. This foundation is crucial for creating an inviting and fluid app. Design foundations from idea to interface.

Each of these starting points is tailored to different aspects of app development and design, emphasizing the importance of context in determining what to prioritize first.