To add a .gitignore file, just create a plain text file named .gitignore in your project root.
Inside, list all files or directories Git should ignore. For example:
Copy
Edit
*.tmp
build/
.DS_Store
Then run:
git add .gitignore
git commit -m "Add .gitignore to exclude unwanted files"
This prevents those files from being tracked in future commits. I usually add .gitignore at the very start of a project.