How can I (in general) program a website for mobile devices?

I really tried searching about this in the internet but came upon diverse search results which barely specified what must be done specifficaly.

I'm now running ubuntu and php5 through nginx, amongst other things.

How is it that I can specify different layout and functionalities for mobile devices under the url "https://m. (...)"?

m for mobile (I'm aware this signature is not always used)