What is the first thing you need to do when joining an existing project that is already in development?

First of all, when joining an existing project, you should:

1. Familiarize yourself with the project documentation (README, Contributing Guide, architectural descriptions).
2. Set up the local development environment according to the instructions (install dependencies, configure the environment).
3. Study the code structure, main modules, and technologies used.
4. Understand the development and deployment process (version control systems, CI/CD, branching rules).
5. Run the project locally to ensure everything works.

This will help you integrate into the team faster and start working effectively with the code.