Put yourself in the shoes of your target audience. What are their needs? What problems are they facing? This will not only help you think of the appropriate use cases, but it will also help you get on the same page as your teammates.

Start with a minimum viable product (MVP). Focus on the core features that solve the primary problem. Additional features can be added later if time allows.

User stories are documented features, described from the perspective of users. They should be formulated in the structure below.

The most efficient and standardized way to formulate user stories is with the MoSCoW method. MoSCoW stands for must, should, could, would. Each category reflects a larger importance, such that must features should be implemented long before could features.

As a [user], I [must/should/could/would] be able to [feature], so that I can [goal].

See also:

TL;DR:

1. Start with creating Low-Fidelity Wireframes: Use pen and paper or simple online tools to sketch basic layouts. Focus on the flow and structure rather than details.
2. Iterate Quickly: Share your wireframes with teammates and get feedback. Make quick adjustments based on input.
3. Move to High-Fidelity Designs: Once the structure is finalized, use tools like Figma or Adobe XD to create detailed designs with actual UI components.

See Wireframing for more information