I want to learn C# for Unity and my personal computer is a MacBook Air. Can I program in C# on a Mac?
There is now a Visual Studio for Mac available as a free community version as well as professional and enterprise versions by subscription.
JetBrains also has an IDE for C#, called Rider, that runs on the Mac. If you use IntelliJ or any of their other products you might like it - it feels better to me than MonoDevelop or Visual Studio for Mac. It is a commercial product but JetBrains offers very generous license terms (as in free) for students and open source projects. They also have a substantial discount for startups.