Learning - Running client scripts based on webservice?

Looking for any kind of advice or heading for me to look into as I'm kind of lost at where to look for this information, let alone try to write it myself.

Essentially. I have a web service that provides data from the backend. I want to create a listener of some sort that runs based on web service returned events and execute back end scripts.

What language or what methods are best for this?

Thanks,