In my opinion, the best way is to study and contribute to an existing compiler.

There are dozens to choose from that compile to javascript: https://github.com/jashkenas/coffee-script/wiki/List-of-lang...

or java's JVM: http://en.wikipedia.org/wiki/List_of_JVM_languages

or .net/mono's CLI: http://en.wikipedia.org/wiki/List_of_CLI_languages