If your iPhone or iPad is on iOS 7 or higher, and you're in the U.S., you've got iTunes Radio. While it's fun to browse stations and create your own, if you're driving or in a situation where you can't launch the Music app and freely browse, you can tell Siri what you want to listen to instead.
That's all there is to it. If you want more control, of course, you can always dive deeper and change things like artist only or a mix of songs but if you want a quick way to get some music going, Siri is a good option.
Note: Apple has said they'll be bringing iTunes Radio to more regions, hopefully that'll be sooner rather than later.