As a convention, if your file contains a single class, name your file the same as your class.
If your file contains more than one class, then the filename should describe the common purpose of those classes. Use CamelCase when naming your files, as per the Kotlin Coding Conventions: https://kotlinlang.org/docs/reference/coding-conventions.html#naming-rules.
Actually, you don't have to write your code in a file for simple snippets. You can also play with the language online: try http://kotlinlang.org/ or use REPL and interactive shell after installing Kotlin and running kotlinc.