Let's start with some introductions:
Google Glass runs Android 4.0.4 and you can easily run any android app  (thanks song for introducing me to Google Glass world). You can run any standard android app, Google just released a specific SDK (the GDK) that you can also use with Xamarin if you want deeper integration with Google Glasses.
Xamarin is a cross platform framework allowing you to create mobile apps (e.g. for android) in C#/.NET will using the native UI framework in each platform.
Now, let's start having fun:
1. Download the standard Xamarin package and open Xamarin Studio
2. Create an Android project targeting an android version <= 4.0.4
3. Disable the system status bar and the application title for your Application with this code:
4. Use standard android controls, buttons, textviews...
5. Try to follow google guidelines if you want to create a coherent experience 
|Screen on Google Glass|
I don't have a Xamarin license to create something much better (because of the limitations of Starter licence) but if somebody gets one for me I'll be happy to build something much better to show the powerful combination of Xamarin and Google Glass :)