You can use whatever file editor you want, but Visual Studio is probably the most popular and recommended choice. It's free, has many features, and integrates well with Unity and Git.
If you're a student, we recommend getting a copy of JetBrain's Rider which adds a lot of code hints and shortcuts that will significantly improve your code quality.
Our overarching goals are clarity, readability and simplicity. Also, this guide is written to keep Unity in mind.
This style guide is based on C# and Unity conventions, and discussions within the development team.
We recently added an EditorConfig file to automate the code review process, it is a file you can add to Visual Studio or Rider and it will apply the formatting and style we use to your analyzers, telling you when something is out of order.