如何使用RelayCommand与MVVM轻框架

I've just started learning the MVVM Light framework and I can't find any straightforward examples on how to use a RelayCommand. For purposes of learning, I'd just like to have a button in my view which when clicked show's a hello world world message box, and which is enabled on every even minute (basically if DateTime.Now.Minute % 2 == 0).I've just started learning the MVVM Light frame