Install, check, and open your first session

Claude Code runs in a terminal, and it works inside the folder you launch it from. That sentence is the whole mental model for this level: the directory is the boundary. Start it in your home folder and it can see everything you own; start it in a scratch project and that project is its world.

So the first habit is the practice folder. Make a throwaway directory with a few files you would not mind losing, and cd into it before you type claude. Everything in this level is safe to do there and nowhere else — not because Claude is reckless, but because you are about to be, and learning where the edges are is the point.

pwd                    # which folder am I in?
mkdir -p ~/claude-practice && cd ~/claude-practice

claude --version       # confirms the install
claude doctor          # diagnoses a broken or partial setup

Install from Anthropic's own setup page for your operating system. This is worth being fussy about: an agent that can read your files and run commands is exactly the thing an attacker would like you to install from a stranger's copy-pasted script. claude doctor is the diagnostic to run when something behaves oddly later — it checks the installation and reports what is wrong rather than leaving you guessing.

Signing in decides which account gets billed and which organisation's policies apply. If you have both a personal account and a work one, the difference matters before you start pasting anything work-related into a session.

Then start a session with claude and, before asking for a single edit, ask what it can see. A first session that begins "what files are in this folder, and what does this project appear to do?" tells you whether the boundary is where you think it is.

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